Thanks for your responses guys, and sorry for the delaying in
responding. I took the cowardly way out and called a plumber. I was
lucky enough to find a superb OFTEC engineer who I'd recommend to
anyone - let me know if you want his details.

So, here's the story of the fix....

As stated in my original post, he suspected the pump as the original
installer (whom I'll also supply details of if required - so that he
can be avoided) had installed it on an angle which I now believe can
cause premature wear. He'd also fitted a larger pump (15/60?) and set
it to speed three - far too much for our small-ish three bedroomed
house, apparently. So, as the re-mounting of the pump hadn't helped,
he replaced it. But, this had no effect, unfortunately.

Secondly, he went to the boiler and realised that even though the fan
was spinning (that's why I assumed the boiler was running), it was only
firing intermittently. In this situation, apparently, the boiler
should 'lock-out' but this wasn't happening. The suspect component was
the control box. He spent an hour or so looking at this and finally
opted to fit a replacement. Two issues were identified when fiiting
the new control box - 1 - the new box hadn't been supplied with a new
PCE (is this correct? Photo- something or other) so the old one had to
be refitted, and 2 - the old box had been wired incorrectly, preventing
lock-out and causing the solenoid to burn up quite badly. So, a
replacement solenoid, too. Still no joy - further testing identified
the PCE thing as the most likely culprit.

So, next day, armed with a new PCE, the boiler eventually fired
correctly each and every time. We now have heat!

The plumber has offered to remove the new pump and control box for no
cost as they weren't necessarily required. However, I'm not keen to
store potential problems for the future so I'm happy to pay for these
and for them to remain. I have to say, too, that the overall bill for
his labour was MUCH lower than expected. He spent 3 hours on the first
day investigating and another 1.5 hours on the second finishing the
job. He supplied all the parts to me at cost price to him, too.

Finally, two further issues - he found the original boiler
commissioning document in the owner's manal. It hadn't been completed
or signed. He's also noticed that the pipework leading from the boiler
is of a narrower gauge than required and both pipes had been fitted to
the same side of the boiler, which means we have no cross-flow. He
suspects that this is why the larger pump was fitted and set to maximum
speed to compensate.

Amazing, isn't it? He's coming back in a few weeks to commission the
boiler (it's been installed for 7 years...), fit the correct fire valve
outside of the garage where the boiler is located (the current one is
actually inside the boiler casing - not much use if there is a fire)
and complete the relevant paperwork. At this point we'll be on the way
to having a correct installation.

So, every cloud has a silver lining. We have a much improved, working
heating system now and an excellent contact for servicing work in the
future. My wallet is a bit lighter, but not as light as I feared it
was going to be!

Thanks again, all.