That really is a piece of string question and unlikely to get a ball park figure except for something ridiculous like £20k. Don’t assume anything and obtain at least 3 quotes. Boiler might be old but working.
IanMcd - a regular very well respected and helpful poster on here regarding heating and boilers. Will be sadly missed.
@stem - thought you may want to know.
I think Conny passed away, I mentioned him on another thread a while ago, and I’m sure someone confirmed it.
Edit, yes he did: https://www.diynot.com/diy/threads/on-line-gambling.589879/page-5#post-6157208
Hard to tell just from pictures and only way to know 100% is get it tested. Looks like plasterboard imo. I don’t think they used AIB unless flatted block perhaps, but could have used Superlux/asbestolux - some people believe this contained asbestos.
Ok. You should have got a tiny trickle at least. Try spinning it counter clockwise with a small flat bladed screwdriver. Probably coincidence that it’s failed. That or you’ve run it dry.