Thanks for all the replies.
They aren't Dartline, i have had a couple of those, or harborough, so we are down to
Swanline by Horace greeaves or
Trentcraft Davison brothers
I have looked online for the publications quoted but failed (as usual) to find anything, does anyone have a link please?
I reckon to have seen half a dozen of these from devizes up to the Trent and Mersey so there must have been a fair few around, the one at devizes had an sr2 but don't know if it was original.
They do look to have low headroom, but the built for comfort or speed question will never be resolved :-)