Best spark plugs to use on a disco 4.0 V8 running LPG

I've checked my plugs to find that one is covered in a black carbon build up I'm thinking this is why my car is over reving when engine is hot I'm being told by car spare shops I need special plugs but am I being told this just so they get my money or can I use standard plugs and gap them to 0.7mm can anyone advise please.

What do they mean special? Why not just try a grade hotter and see how you get on?

Or could you be burning oil on that cylinder if it's only one?

Special as in ones made for LPG there about £70 a set I had issues with the leads which I think may a caused the issue with the plug I've now changed the leads so I'm hoping that will sort the problem.
What do you mean be a hotter grade ?

Hotter means a plug that run a higher temp in the engine during normal running.
So instead of (say) a bp7es you use a bp6es to reduce fouling. However, it's not a cure for engine faults and I don't know anything about special plugs for LPG.
I suspect these are just more exotic standard ones like iridium or surface fire.

What exactly do you mean by over-revving?
Does it do that on both petrol and LPG?
Is it a single point or multipoint LPG system?
How is the LPG controlled - is it interfaced with the vehicle's own ECU or its own?
Usual suggestion for plugs is NGK to the plug manufacturers specification for that vehicle. TBH I can't see the plugs making any difference, its likely to be a fuel/air mixture issue.

It did it on both lpg and petrol I've put a set of standard plugs in and it seems to have stopped the problem it's just not got as much power as it did but at least it's not reving like mad when I'm driving.