possibly currant blister aphid.
link to the RHS site below.

https://www.rhs.org.uk/advice/profile?pid=491

certainly looks typical of insect/mite damage whether it is another critter I don't know.

I get blister mite lke this every year but not with the red patches.All I do is to cut the affected parts off and dispose of them safely (used to burn them but we are not allowed bonfires at present) Once removed it doesn't normally happen again the same season. You would normally trim redcurrants anyway and I've never found it affecting next year's crop of currants.

Thanks Stera, I've never grown them before and my neighbour is an enthusiastic novice who is delighted that anything has grown in his garden bless him. I will tell him what you said.

Tell him to keep cutting them off at the first sign earlier in the summer - seems to lessen the amount of spread.(I didn't do it this year and my bush is a mess - serves me right...)

Thanks, will do.