Thanks guys for your input.
Sorry I didn't reply sooner, no Internet and very poor 3G at my mums (no skirt waiting !)
I'm afraid I chickened out of the scenic adventure and went the boring M4 route when I saw it was pretty cold first thing on Tuesday.
It was a 242 mile trip across there, so by far my longest run in a day in my 9 months of SV ownership.
On previous runs of decent mileage I did suffer wrist ache and the usual lower back/a**e/leg pain, so I invested in a cramp buster off amazon to see if it helped - it really does work, no wrist ache, best £9.99 I've ever spent
Just remember to rotate it so the tab is at the 6 o'clock position for roundabouts etc !
I wore some padded cycling shorts under the layers so managed a couple of 90 mile runs before I couldn't take the pain any longer, and needed the solace of coffee & sofa !
Returned yesterday in constant rain, cold & strong wind so once again I chickened out and took the M4, the rain stopped once I crossed the bridge, still cold & windy though.
I hope to do the journey again in the near future hopefully in something resembling a summer. I will definitely take the scenic route next time.
Thanks again all.