pressure dropping from my worcester 230 combi boiler?

If it isnt the system drain off valve that's leaking Ronald, then it could be the boiler thermostat has gone and is causing local boiling and the relief valve to operate or alteratively the valve maybe defective.
I would call an engineer for the latter two. If the pipe has no valve along its length or on the end of it, then it has to be the pressure relief. Trace it and see.

Bucket head
December 2010