Just wanted to update those of you who remember me posting about this,
and a big thanks to Ed and everyone else for their input.

My problem was that my DFE fire was cutting out after a few minutes of
use.

When BG visited to service my boiler I got one of them to look at it
briefly and (aside from banging on about the ventiliation
inadequacies) he also said that the pilot flame seemed too small (I
think Ed mentioned it needs to envelop the thermocouple).

So off he went. A few days later I decide to have a look-see myself.
Pilot light outlet simply pulled off the gas jet without too much
force. I gave it a good blow and clean with a toothbrush, and also the
gas jet which didn't look too bad. Put it back together and when I lit
it the pilot is now massive!

Fire stays lit all night, so job's a good 'un, only took 20 mins to
do... unfortunately the nonce that fitted the fireplace (before I
moved in) didn't fit the grill properly so I spent the best part of 2
hours with my head in the chimney trying to get some tiny nuts on the
end of the short bolts that came with it. :/

Just one thing: do you think I should get an air vent/brick fitted? BG
quotd £60 (but not sure how ugly it looks), I was told it needs to be
125cm2 in diameter, which seems potentially drafty to me. I know the
fire is classed as 'at risk' by BG because it's not an inset one,
though the pull on the flue is fine by all accounts, I just don't know
really.

Cheers,

Stuart.