If its popped up on its own, do you really have room for it? Common holly gets upwards of 12 metres with a spread of 4-8 metres, but if you really want to keep it, the time to attempt to move it is late autumn or in winter. If the move is unsuccessful, I have no doubt you will find other volunteer hollies popping up anyway.

Thanks Bamboo,
the idea of moving it was just the fact it was self set and looks healthy, it was my intention to try and keep it as a smallish (easily manageable) standard . If all fails no great loss as a freebie

There'll be others, likely not far from where this one germinated - the trick is to find them when they're small and easy to relocate or get rid off!