Thanks, and to @dangeek. Reminds me of a bad experience with a builder a few years back. I’m sure they have plenty of happy customers, but still boils my piss when I think about them :D
I’ve had frames nicely restored by Argos. Nothing is ever quite perfect, but I also think that is due to starting material - 50+ year old frames that have been used are difficult or at least very time consuming to get back to perfect. Funny that vintage bike restoration attracts ocd bike geeks (me included). Like moths to the flame.
I am still leaning towards Mercian, just seems appropriate to get an old Mercian restored by them. I had a very pleasant phone conversation with them and they are also doing free collection and delivery if ordered in June. Guess I will just beg for a careful job upfront and promise a good write-up if I get it!
Thanks again for your advice.