Thanks Carleton, great advice so I went in with kit, shoes and pedals and took some photos:
Large:
Extra Large:
The XL felt better. Didn't feel too long, although the bars may be a bit low but I can flip the stem and will probably replace the 38 bars with something a bit wider and possibly less drop.
Presumably the saddle-bar drop for a given saddle height will be less extreme on the XL
The XL cranks are 170 which is probably a better bet given I use 175 on the road
Both bikes have 130 stems - I figure better to shorten the XL stem than lengthen the L.
With that in mind I've taken the XL, but if you think I've got it wrong, I can swap it for the L next week.
Thanks for the help and advice!