Morning Masma,
Being that you have tried to get it repaired and it has cost you a small fortune, i would be inclined to not spend anymore time, effort and money on it............shop around for a good deal for a new boiler.....you may be able to exchange yours in return........and look for a warranty, maybe ask for it free for 3 years to start, so you will be covered if anything was to go wrong with it.( some companies will negotiate a deal.....remember it is a buyers market out there, as everyone wants your business)
Companies like Heat team do a policy where you can cover the boiler for around £120 a year, after that....all parts labour call out etc.........and that would give you peace of mind too.
You could spend some time looking on forums to see if anyone had/has the same fault as your boiler, but if it has been looked at, it may be just unrepairable.
Good Luck to you.
yogi