The boiler terminal block is behind the panel where the boiler thermostat is, it wont be in a 5"*5" junction box.
you can access the gas valve via the bottom panel, to check for the voltage.
Finally found an expert well 1/2 an expert - read below.
Doh! I was thinking you were talking about the mass of wires and screw connector blocks from the valves to the boiler. Silly me.
Now 175v DC is the ignition feed I take it.... .
Called British Gas in the end - if they can't fix it nobody can - I thought.
Well... Guy turned up @ 15:15 and checked the system - p****d about until 18:00 and decided he needed a better class of expert to help him. Left us with No CH or HW Monday night. Actually he made six or seven phone calls to someone and I heard him ask about what the valve wire colours do and another saying that he could only get 150V.DC - obviously he needed the 175V.DC but it works now.
Turned up again @ 10:00 mob handed (3 blokes). Dived into the system and came to a similar conclusion as myself - the guy, 3 owners ago, was a total loser.
However, after a couple of hours they traced the intermittent fault to ----------- a dodgy NEUTRAL.
They rewired the Neutral and the HW & CH valves - re-checked everything and left.
Now I have (or I hope I have) independent HW and CH a full running boiler - apparently nothing wrong with the boiler and controller (they swapped that first and put my old Landis & Stafea unit back as it's better than the British Gas one).
I'll update this tomorrow as I want to wait to see if it packs up tonight at 20:30 as it has been doing.
Thanks for all the help and remember - dodgy NEUTRAL - we learn something every day - well these 3 guys said they did.
Steve