Thanks for the replies, I'll go with 25mm. (Wickes is cheapest I've seen for a 25m roll).
Yesterday, I excavated down to the water main at the boundary. I found a buried stop c*ck (there's a nice new one in the road), and the pipe at that point is 7lb lead, and looks to be in good condition, so I'll probably cut through the lead at the boundary and run the 25mm MDPE from there.