I've had this before, the draught is probably coming in through the drainage channels at the bottom, it then works it's way around the gap between the slab and the frame and comes out at the only place that there is no brush seal which is the strike plate get hold of a bit of adhesive brush...
I have done these and they're not easy I get mine from warwick developments (spectus I think) cill comes cut to size as a separate item and sliders come with stub cills but all holes are pre drilled so you get the welded cill cut to shape and in position then shuffle it around to get sliders...
as an update, just built a new sunroom, instead of using equinox roof I made my own with timber 6x2 then bought insulation, membrane, plywood and tapco composite slates, insulated plasterboard inside and skimmed. Exact same look as equinox, same u-values but no danger of cold transfer because of...
ive fitted similar roofs from eurocell their system is called equinox and I too enquired about just getting the aluminium frame but not a chance they can make a big mark up on these kits branch managers get good incentives for selling them as these replacement roofs now come under building...
I would always use fixings if you've ever tried replacing windows that are on cleats then you'll appreciate why, personal choice really I cant stand cleats
not too hard to fit its whether you come across anything else that's where the experience comes in but I would normally charge about £90- £130 for that job depending on cost of lock :)
I have done this on a bay window with quadrant corners I made a template for bay then took it to a local firm who specialise in arched pvc window manufacture and they welded up all the pieces and I fitted it still works fine and looks good but this is Grimsby so don't know if anyone does it near you
you must have a local supplier or a local window company who would come out and measure up and supply only for a small charge don't bother with b&q or wickes or anyone like that
woodgrain type fascia boards are usually colourfast but brown upvc isn't good as for guttering same applies, brown isn't perfect but should last ok why not black guttering on woodgrain fascia ?