Thanks all for the comments, for any one else reading, this is my advise.
If like me your plumber fitted a 14inch immersion heater in to your 1850mm tank, even if it was only several years ago, if it's not leaking do not touch it unless your boiler has packed up and you have no choice!
Do not listen to anyone stating its an easy job, you have no idea what damage has been done and the threads and boss are incredibly light weight.
It would appear my plumber did not remove enough insulation around the top to make way for the washer to protrude around the flange.
it came with the original thin washer and he must have arm wrestled to pull the immersion to the flange, partly compacting the insulation (but it doesnt compress well at all) and resulting in pulling the delicate flange in to a warped state where the flange was then raised on 1/3rd of the area.
My first mistake was cutting back the insulation so the washer sat properly, i may well have got the original immersion back on again as the insulation was providing the packing for it to mate in that unlevel state.
The threads were not cross threaded, but I could see they were a tiny bit shredded in 2 small areas with pieces of copper filings where it must have been over tightened.
When tightening it screwed in for several turns with no resistance and became very tight, if it ever gets tight wind it back and check it is not cross threading, then keep refitting by turning anti clock till it locks in place. In my case i kept doing another flat, winding it back and continously checking. Once i got 3/4 of the way it was incredibly tight and you'll see on the pictures it was because it was warped, then going past this point it caused the thread to jump off, when undoing there was no cross threading but it's because of the force of it trying to mate a flat surface with a warped one, the copper threads are weak so it just jumps off.
The solution I performed:
Buy some nice, thick, immersion washers. I bought 3 from toolstation, they are much wider and thicker than the thin, breakable fibre washer that comes with the immersion.
Buy an immersion blanking top, I got mine from
screwfix and its a nice strong brass one and some lsx leak sealer.
Clean the mating surface 100% of all broken fibre washer and whatever gunk your prior plumber has put in there.
Clean the boss threads with an electical screw driver, ensuring the whole thread is 100% clean of any debris.
Insert brass immersion cap with small amount of copper slip on boss thread , start threading this in and any small resistance go back and lightly forth to help re-cut any threads that may not be smooth, all by hand. In my case after a turn or so it starts gets very tight as the boss is warped.
Get your spanner on the immersion cap and start going back and forth very slowly, easing it down.
After about half way mine was now incredibly tight, the boss is only soldered so if too much force is given it will snap.
Once you have gone as far as you can, back it off and do it a few times
In between I used a small hammer to lightly tap the raised area to try and encourage it flatter (its difficult to see if this helped)
In my case i could not get it lower than a 2 washer thickness. You are better off packing it thicker as there is less strain on the threads, you just need it to mate together under compression to perform a seal.
Fit your new immersion with 3 new, thick immersion washers and see how low it can go and how flat.
By now i had rectified a lot of the warping and from what was a difference of over 3x thin washer difference on one side, i had got it to one. I.e it was flat on one wide but i could shove 3x 1/3rd's of washer stacked.
Remove immersion, put lsx on under side of washer to immersion and then liberally on top of washer where it will sit against the flange before going for the final run.
Insert your thin, cut up to size of warped area fibre washer on top of your 2-3 washers on the side that is elevated, put some lsx on to stop it from moving and keep pushing that thin washer back in to position as you tighten.
In my case i tightened it more than I would have if it wasn't warped, but this allowed me to have the fibre washer completely mated the whole way round, there was no flex.
Leave the lsx to cure for 4 hours before turning on the water.
Open tank fill gate valve and constantly check for leaks, I left mine for a whole night before I boiled the water hot so i knew the lsx was 100% dry.
In my case after reopening the gate valve i was not getting any fill in to the tank. Aftet subsequently draining the tank partially later via the drain, it started filling again. In my case there must have been a bit of dirt get sucked in to the pipe when it was draining or some kind of air lock.
Me personally ,I am going to buy and fit another tank for piece of mind, but at this point large tanks are taking minimum 2 weeks to order.
Good luck
