I'm running a x-flow with 10.2:1 compression.
My current plug is a BP8ES it seems to run cold as the plug is dark coloured (blackish) but the Lambda readout of the mixture is correct 14:1.. is there a guide as to what type of plug to use or is it trial & error?

try
http://www.ngk.de/Spark_plugs.649.0.html

An NGK '8 'is a very cold running running plug, unless your engine is in a fairly high state of tune this will almost certainly be too cold.

As a guide the standard grade for the Kent is BP6ES, and Caterham used BP7ES on their 1600 106bhp Kent engines.

well a 244 cam is running in this engine, what state of tune would that make it?

Black deposit is probably caused by oil however BP8ES is a cold plug for an Xflo I would try BP7ES

BP8ES = Champion N6Yc
BP7ES = Champion N7YC or N8YCc
BP6ES=Champion N9YC

It doesn't look like oil, thanks for the plug advice will try those out