ISAR HE30 BOILER PROBLEM?

My 86 year old Mother had a Ideal HE30 Boiler fitted under the Warmfront scheme in July 2007, nearly 3 weeks ago she had her bathroom papered, the heating was fine when on, then she was away a couple of days so we then turned it on for her and noticed she had no hot water, hubby looked at boiler and noticed a reading L9 , we switched it off and rang the hot line who asked him to switch it back on, a bar came up on the symbol straight line the pressure gauge started to pulsate and water strated to leake from boiler at back she told him to close it down and she would send a engineer out.
Next day he turned up and before he got through door it was obvious he did'nt want to be here, he went straight upstairs opened airing cupboard door he could see that the bathroom at been decorated and straightaway said you have decorated and removed the radiator and blown the boiler mate, my husband made it clear that at no time had the radiator been disconnected, he said well you're blown it, my husband asked him to explain how and he just repeated himself.
He was that ignorant that my husband had to walk away and l telephoned my son to come round and wait with me, l watched him for 29 minutes unknown to him all he did was stare at the boiler prior to this he just fitted the link pipe and said there no water in the system and the compustion cover not securable 1/4 nut dislodged and cannot close chamber (the only people ever to have the cover off are the engineers when serviced) , my husband told him well it's leaking out he did nothing else.
He left and when l telephoned to see what was going to happen l was told it was down to us moving the radiator.
We complained further to the complaints dept ran by Eaga? they said it was not honoured because the decorator was not Corgi registered, nor was the lady on the other end of the phone when she told us what to do.
At this we contacted our Home ins company who organised yet again for Homeserve to come out they did give a very good inspection and came up with the idea that it was the Air Valve so they ordered one and fitted (the old one was covered inside with corosion) but then they came across a leak which now they say that the Heat Exchanger needs replacing with a cost of £400 plus but more than likely will need a new boiler at a cost of £800 -1000 fitting plus the boiler.
This boiler had 2 years warranty and was also extended, we have no idea what is going on, neither my hubby or the professional decorator did not move the radiator there was no need, l would be grateful for any guide any kind person can give me.