Craig,
The Civic engine uses 32 mm ID pipe, the rover rad is 28mm OD. I've got samco 90 deg reducers on to the radiator, then a mix of 32mm OD 316 stainless (with welding beads, cos i haven't flaring tool), and 32mm ID hose / 90 deg bends. It's very tight under the gear lever, but workable.
I ditched the plastic guff, as the ID is very small, and the last thing I want is a marginal cooling system.
Any good hydraulic hose supplier should be able to provide suitable hose / pipe. I used:
http://www.hampshirehose.co.uk.
It cost me about £60 to do the lot (samco reducers were quite pricey, but I couldn't see a decent alternative).
I think Jerry went to town on his, and bought a crate of samco, possibly from
www.merlinmotorsport.co.uk (pretty close to him).
Cheers,
John.