In reply to gazfellows:
You'd be doing everyone a big favour if you did remove it, Garry - that includes climbers, walkers (and sheep!). If you haven't done this before you need to be extremely careful, both setting up an abseil over the line and then removing the rock - a flying boulder could really spoil someone's day down by the tarn! At the very least you'd want someone below, out of the line of flight and able to warn people off while the job's being done. It's a potentially serious situation - one to bring to the attention of the National Trust who (I think) actually own the land around Stickle Tarn? They'd definitely want a dangerous block taken care of and almost certainly not want an 'amateur' getting stuck in.