Thanks everyone for replying to the post. I have been busy chasing quotes the whole day.
This condemned boiler, here's the story: so, we have had British gas (BG)Homecare contract for over 4 years which includes among other things, central heating and boiler service. On 2/11/22, the BG engineer came to service the boiler which was working perfectly ( 22 rads, loads of hot water). About 3 mins into the service, he called out alarmed, asking if the boiler had ever been serviced. I was confused. I told him it's been serviced annually by BG! He showed me a hole in the casing and pointed to a horrible looking yellowish crusty weepy joint on top of the boiler; apparently it had been leaking over a long time and it had caused corrosion on the case. On close examination, the whole area behind the flue is rusted. This is a room sealed boiler and so it's an integral part of the boiler. The leaky joint is clearly visible when the front cover is off - it appears prior BG engineers couldn't be bothered to fix it while certifying the boiler as safe. Might I add that the carbon monoxide alarm present has never detected any gases. British gas denied responsibility for negligence and say that the boiler was old anyway but as a Matter of "goodwill" they've offered to refund 2 years worth of boiler service fees we paid and also one central heating cover totalling - which is still somewhere in the post.
Now we've got about 5 local plumbers in but all of them say it's unsafe and only quote for a new boiler.
I think we are now tending towards a system boiler as the hot water was so slow to get to the tap and we have two showers with 2 pumps - like I said we have 2 separate plumbing systems (original house and extension. Well get it combined. We'll be renovating but with high prices we'll wait So perhaps in 2 years time. For now we'll upgrade the mains - we'd thought we'd do it with the renovation but it is what it is.