I am unsure about a kit still being available, but there are a number of plans for the Dark Class FPB.
The original plan is MM564 by myhobby store and soon to be available from Sarik. The model from this plan is approx. 790mm long and is typical hard chine construction. The original magazine item was in Model Maker magazine October 1956. A glass fibre semi-kit was released in the 1980s and was based on this plan, it was sold by Hydra-fibre who are no longer trading. The kit was reviewed in Marine Modelling International magazine March 1989.
A plan was also sold for a 915mm model by Marine Modelling International in 2014. The plan number is MAR3677 and the accompanying magazine item was in the March 2014 MMI. Again this should soon be available again from Sarik.
A 1:24 kit of Dark Biter was sold by Nautical Marine Models until recently and this was reviewed in Model Boats magazine March 2011. Built by Dave Abbott, who had previously won a gold medal at the Model Engineer exhibition with a Dark Class model which he had scratch built from plans obtained from the National Maritime Museum. If the plan from the NMM is of interest - take a stiff drink before reading the price!
Tony