daytonaRS wroteA chap on here, HAGA 40, from Holland makes a fine alloy sub frame, as per original. Supplied with Ti bolts. Very good price. I'm very happy with mine.
Just got my 851 mono from Haga40 as well, cracking job. An honest review is that initially it was wrong in a couple of places so Ger has had it back and corrected the errors. Although I've only had a moment to fit it on the frame and pop the seat on top the attention to detail is great along with some very nice welding. Mine has been made with a stronger front tank support to help with the rigours of the Classic TT for this years 2nd bike Katy.
See
https://en.wikipedia.org/wiki/Austin_K2/Y for the this years name explanation and the film it comes from.