Most likely covered a thousand times but
What spark plugs for a 400BHP coupe (20vt)

1001 times now

Depends, but many fit 7's as it's a cooler plug.

If you're using quite a lot of boost to get the power, consider NGK BPR8EIX

As they say theres many ways to skin a cat

I use NGK BCR8ES which are tried and Tested by a few of us and are cheap enough (unlike the iridiums). I always check the gap is set at aswell becuase ive have had a few plugs that have been knocked in the post.

I tend to gap mine down to about 0.6 - 0.7 mm, as I find that the spark can be extinguished at high boost

I have these in my 16vt Nigel. Should I be running something else?

Unless you're highly modded, they are probably overkill, both spec-wise and price-wise. Way too "cool" for a standard engine

I haven't managed to find out a lot of history on the mods made by previous owners.
I know it has a decat and a chip but unsure if anything else.

In which case, eight-grade plugs are probably too much - sevens or even (standard) sixes will be enough

The eights are more likely to foul, as they simply won't get hot enough unless you're driving hard

I've had them out a couple of times this year and they look fine.
I do like to stretch its legs quite often though.

I use 1.4-1.5 bar. Thinking of buying these, but the price is quite high.

http://www.sparkplugs.co.uk/ngk-iridium-spark-plug-bpr8eix?___SID=U