Skimming a radius corner

I'm hoping to skim a lime/sand/cement ceiling in a mid 1930's shower room of about 2.7m x 0.85m. The ceiling has a skeiling running the length of it with a radius corner between the slope and horizontal of about 150mm to 175mm. I'm happy with skimming the flat sections but the radius area is something I've not done before. Could someone give me some tips on doing this part or point me to where I can get some help, for instance do I use a special trowel or will a normal finishing trowel be OK?

Many thanks

Show us a picture so we can see it,, much easier than describing.

Here's a couple of photos and a layout diagram of the room. I hope the dark patch in the skeiling area shown in the first picture is left over damp from the steamer used to soften the original paint/distemper/textured finish although this area previously showed signs of mould. For reference the tiles are 6 inch.

That looks easy enough to do. You could do that with a normal plastering trowel, 11",, 13". It will prove tricky if you've not done this before though, and it's difficult to explain. I normally do this type of ceiling/wall in 2 parts. First, I'll do the skeiling, starting below the radius and working along the join where the skeiling meets the vertical part of the wall. You'll need to prep the skeiling/wall first anyway, Once prepped, (all loose material removed/wall sealed, cracks taped etc), you'll need to pva the skeiling, and plaster onto it while the pva is still wet/tacky. Apply the plaster with the trowel onto the skeiling. Bearing what I have said in mind, go online and type in, HOW TO PLASTER A BARRELED OR CURVED CEILING. Up will come a Youtube video of a plasterer, plastering a ceiling, more or less identical to what you have. Watch it and you will see what is involved. It's not as easy as he makes it looks, so beware.
Going back to your photo's, i'd be very wary of that dark patch on the wall. I think that's "damp". I reckon that over the past 80 years or so, rubbish, dust and other material in the space above the curved ceiling has jammed/gathered at the bottom/edges of that slope behind the lath and plaster, allowing the damp from the outside wall to penetrate through the plaster. In other words, the gap that should be behind the lath and plaster has filled up with dust, sand, whatever, causing the damp patch you can see. Anyone else agree guys. Another thing, if you plaster onto that area as it is, I think you will have problems with it.

yeah id be going up into the loft to see if the eaves in that area are blocked,maybe using a garden hoe so you can pull out any crud?.
if it turns out to be really blocked then you might have to pull that part of the skeiling down?
of course if its clear then other avenues need to be looked at.

Thanks for the info, very much appreciated.

I put a dehumidifier in the room for a while and the damp patch appears to have almost dried out but I will not do the ceiling for a while to see if damp creeps back. Perhaps a bigger problem is that yesterday it lashed down with rain and 3 damp patches appeared on the ceiling toward the other end of the room. These new damp patches are in an area that has a history of dampness due to some problem with the valley above, which has been fixed twice in recent years.

I take your point about the debris above the skeiling but getting in the loft is a no no cos the only access is almost impossible to use due to the small opening, low roof height and valley right next to it. Being that the roof is the original Mid 1930's construction it has no underfelt beneath the tiles so perhaps the easiest option is for me to get up on the valley and remove a few tiles to see if I can see what the problem is.

I would agree there is definitely a problem there which relates to one rafter section there. Judging by the way it appears to eminate from along the wall and then down into the mansard I would suggest that its running along the ceiling.
I would dry it out and see if it comes back after heavy rain again or whether it just reappears, if it just reappears then it could indeed be blocked with something.
To be honest I think you have enough issues with other water ingress which you really need to sort first. I don't think lifting tiles is really the way to go.
Probably not what you want to hear but if you can't easily assess the root of the damp issues from above then I would consider pulling the ceiling down, getting a look at what is going on and fixing it, reboarding it and then plastering it. stick some insulation up whilst your at it.

Pulling it down would be the way I would go too. Also, what about the vertical wall on the right of the picture, is that lath and plaster too, or a solid plastered wall?
All this talk about pulling down the old ceiling, and maybe a wall too, is not as bad as it seems. You're more or less saying that you are not bad at the diy, so I reckon you could do the vast majority of that work yourself.

Trouble with pulling the ceiling down is that it isn't going to help if the problem is broken/slipped tile(s) or a split in the valley tray or a build up of debris on the outside of the valley not allowing the water to drain away quickly enough. The vertical side wall is solid brick or blocks; the other side of the wall is the bathroom which has also had a damp issue. Just been told that the lead valley tray had a crack in it about 3 to 5 years ago that was patched; perhaps the patching has failed. As it happens I've still got a half 25 ltr pot of Hyflex Hyvodex 30 roofing repair solution I bought years ago (works brilliantly at stopping the bottom of wooden fence posts from rotting when there concreted in the ground).

As for my DIY skills you're kind of right, I work in the building as a sparks for some 30 years and used to do a lot of associated building work myself, that is until Ill health kicked in; it's as much lack of confidence these days. Anyway because access to the valley is difficult (hip roof one side and gable end the other) my friend wants me to contact the roofer again so I might be off the hook for the roof problem.

Is this forum the right place to get a guide price to strip the roof, install an underfelt and refit the tiles?

Yep Wobbly, getting the outside/roof watertight is definitely the priiority. As regards to quotes and prices, get 2 or 3 locally, from reputable roofing firms. Good luck anyway.

Agree wobbly if there is an obvious fault in the roof covering valleys etc but if its a difficult one to asses from above then investigation from below might reveal the issues