Now resolved. The drive replication required access to was on the C: drive of the distributor.

I can't see why it would need it for this specific publication/subscription and not for the other 4 transactional and 4 snapshot publications using this distributor, unless it was down to size as the snapshot for this one was over 35GB.

This does add an extra problem as this is a virtual instance on a cluster so I now have to give the account access to the directory on the C: on all 4 cluster nodes.

any suggestions anyone?

Thanks