I have just checked the last readings I took and the boiler thermostat cuts off when the flow temp is 87C and cuts back in when it is 54C, return varies between 43C & 51C, do you think my boiler thermostat is knackered?
That is a very wide differential on the boiler stat and, according to the manual, they should not go above 82C; so it may be knackered.
can I fit a room stat and turn the boiler stat up to full, or should I replace the boiler stat as well?
If you fit a room stat, that will effectively control the boiler. Which turns the boiler off first will depend on which heats up quicker - house or boiler.
No harm replacing the boiler stat.
should I just buy a new condensing boiler !?
That depends on so many factors that no one can give you a definite answer. But, if the boiler is working or not too expensive to repair, the general advice is to keep the old one as the reduction in your fuel bill when you install a new boiler is unlikely to cover the cost of financing a new one - unless a few £k is just small change in your pocket.
Though with the current economic climate and interest rates very low ...
But don't forget that £3k in the bank is there for ever earning you money, if you want. But £3k spent on a boiler will only last the life of the boiler - 5 to 10 years - after which you have to find another £3k or £5k, depending on inflation.
Am I correct in thinking that I shouldn't fit a room stat anywhere that is controlled by a TRV (which is the whole house at the moment)?
That is correct.
If you have a room stat and a TRV in the same area they will conflict. If the TRV is set lower than the room stat, the room stat will never turn the boiler off. If the TRV is set higher than the room stat, the room will never reach the temperature set on the TRV.
You should either replace the TRV with a lockshield valve (prevents someone turning the rad down) or just take the top off the TRV so the valve stays permanently open.