Thanks guys. Well, it was the venturi!!
We were going to remove the fan to clean it, and it seemed a right ballache with the condensing assembly needing disconnecting to remove the whole top half of the assembly. (and then the boiler needing isolating and draining down ... leading to possibly leaky isos etc) .
Next on the list we tried to undo the 4 screws underneath the fan (tight in above the heat exchanger). Try as we might there was no way the back two were coming out even with a small ratchet driver.
So I ended up blowing through the venturis using some ugauge tube, and then I got a small tie wrap (actually I sliced a big on in half, lengthways) and used this to clean the L shaped venturi.
it worked a treat and went right around the corner and after that I blasted them through with a bit of air again to be sure.
Bolted it back up and she fired up a treat, and ran pefectly!!! RESULT.
Not heard anything back from the custard so it's still working perfectly (they WOULD have rung back..!)
So big thanks to all those who helped out with this bugger, and hopefully this thread will help someone out in the some boat at some point 🙂
cheers