Since my original posting asking what a Jostaberry is ... I think we've got one.
When we first moved here we bought a job lot of gooseberry, redcurrant and blackcurrant bushes from a village hall, none of them had labels on. One of the bushes has always been a bit of a mystery as it was sort of like a blackcurrant but the fruit are larger and a different taste. Just thought we'd got a rogue blackcurrant
Have just been outside to have another look at it and pictures on the tinternet and yes I'm pretty sure it is a Jostaberry.