I use premier gas in Wigan, they installed and service our boiler now. Highly recommended.

first thoughts straight away from being a plumber is the pin is stuck in the trv take the head off and see if the pin moves up n down if its stuck try some pressure onit to loosen it hope this helps let me no

The Engineer has just left.... Still a mystery :S
The pump is working fine.
The boiler is firing on demand but is only firing for 90 seconds maximum. The engineer has tested the overheat stat and flow stat which are working fine.
He seems to think it is a small blockage either between the boiler and first rad or inside the heat exchange within the boiler. He is calling back tomorrow to inject a de-scaler into the system.

I know one thing.....its feckin freezing LOL

freezing isnt the word mate lol, im on site doing complete pipe ups on new builds houses are freezing no windows or doors just a timber frame and some plasterboard lol i carnt move my hands ive got so many layers on these conditions are unworkable haha right so like i said before thoughts straight away was the TRV change it there only a couple of quid from a tool station then you no for sure this isnt the problem, now you say your boiler isnt firing for longer than 90s i would put this down to a gas valve problem which is commen with worchester boilers or your boiler isnt getting enough air to combust, cause its cold there will be a drop in oxygen co2 etc and your boiler needs all these to cobust properly if they dont thats when you start gettin your carbon monoxide problems so is your boiler in a kitchen in a cuborard or just on your wall in the kitchen, try leaving the back door open for a bit then fire your boiler and if it works you no that its just a ventilation problem so just get another vent put into your kitchen, had a few what have done this the first one we couldnt find anything wrong with the boiler when we was working at the house we had the back door open and everything was working so when we went the back door was shut so it wasnt getting enough co2 to combust it uses whats it the room then goes out and we was pulling our hair out cause we replaced everything then we shut the back door just by chance and then we knew it was this so like i say mate TRV if one rad isnt working but the rest is and if your boiler is lighting then going out either a gas valve problem or a ventilation problem hope this helps mate let me know thanks.

Cheers for your reply Wire 91.
The TRV's are definately working. I've had the radiators off and they are opening and closing correctly when the rads are off.
The boiler is upstairs, it has slight flow so indicates its only a part blockage. The boiler isnt staying lit because the stat inside is saying the heating is at temperature therefor the rest of the system isnt getting warm( i assume without ripping the boards up that the cold rad is the last on the system.

We've added a descaling chemical today and will add a sludge remover in the next few days.

I'm pretty sure its not a ventilation problem cos this fault has only just started after renovations and the installation of 2 new radiators. I'm 99% certain its a blockage somewhere within the system but i'm just a thick spark

I know what you mean about site work, we're on new builds at the minute, same as you. First fixing in this weather is criminal!