Reply to John:
Yes, I'l be picking my 'Victoria' plums soon, but I think you may be confused about what summer pruning plums is all about.
I'm not sure what advice you refer to, but please always refer to a reliable source, like Gardeners' World. Summer pruning simply involves shortening all new leaf shoots, and does NOT involve cutting off stems carrying fruit.
By pinching out the tips of all new leaf shoots through the season back to about 5 leaves you encourage the buds at the base of these shoots to produce flowers in future years.
More major branch pruning can also be done to large trees over summer, but this could clearly remove some fruits.
The reason plums are only pruned in summer and not winter is to avoid spread of silver leaf disease. This disease can spread into wounds and pruning cuts in winter, but for some reason isn't a major problem associated with summer pruning.
I hope this helps.