We are on the same Isotherm as Newport in Shropshire, which had the lowest recorded temperature in England. So far in the last 8 years the only monthin which we have not had frost is August (missed it by 1 day last year). We have an unknown greengage in the garden and a hedge row of them just outside. So far we have had 2 good crops and 1 poor, the rest of the time no fruit at all. The deciding factor seems to be when you get frost. If it occurs when the tree is in blossom (and this includes windchill which takes the temp below freezing without causing white frost) then no matter how hardy the tree.....no fruit set. If frost arrives after fruit set there does not seem to be a problem (unless it is very severe).
So, what you should be looking at is a. when the tree flowers and b. when your last frost usually occurs.
Also remember that if you plant halfway down a slope the frost tends to sink past into the lower level.
Hope you can find a suitable tree and place, green and red gages are delicious.
