Found accidentley two larvae, after looking in the Readers Digest book they match the lesser stag beetles. But they are about 25-30mm. Is there a difference is size of larvae of the Lesser Stag Beetle and the larger Stag Beetle. Both larvae returned to the rotten log they came from.
Lesser stag beetle larvae are generally smaller than stag beetle larvae which at their largest are up to 80mm long. However, you could have seen a young stag beetle larvae. Both species can co-exist and are associated with dead wood, though with regard to behaviour, stag beetles prefer to feed underground on dead wood, whilst lesser stag beetles are more active overground.
The link below is very informative on various Scarabaeoidea larvae