Thanks for the suggestions of the doublers Tim & Bob, we aren't using Gigabit (at least yet), so we only need 2 pairs in the CAT5. We could split off two pairs...
...but there's no network socket in this new location to split! :-(
Looks like it'll be new trunking, unless a kind soul can point me in the direction of something to suitably insulate the CAT5 inside the 16x25?
Bob, you mentioned some "mains grade flexible trunking".. Do you have a link with a suitable product? I don't remember seeing this before?
Thanks for the pointer to this in the regs from your earlier message, my brother has a copy of these I'll ask to take a look at today.