Sorry to disagree with some of the other posters Martin – but I think there is a better solution for your friends 'Derwent'.
As you will (probably) know, 'Dewent' is a small Gauge '3' locomotive designed (by G3 member Dick Allan) for garden railway (e.g. scenic) operation. I've seen Dicks' version of this engine run a few times at G3 GTGs (under radio control) and very nice it is too.
So it would make much more sense (at least to me) if you used the Gauge '3' Society's wheel standards. You can either download them (as a PDF) from the G3 Society's website – or if you prefer, just PM me and I will email them to you..
Your friends engine will still run on 2.5" MES tracks (should he wish it to do so – although it's clearly not a passenger hauler) but the engine will also be able to freely navigate the turnouts and other track features at any Gauge '3' Garden Railway – if he ever decided to give one of our GTGs a visit.
We are always happy to see Guests (and their engines & stock) at our GTGs but as they are held in Members Gardens, we do ask for people to contact us (for an invite) beforehand – rather than just turning up.
You might also tell him that the Tea and Cakes at our GTG are also usually very good too! 
Regards,
IanT
(G3 Member)
Edited By IanT on 05/08/2015 14:06:58