I see the bit about 6m head. Are you saying not to get a grundfos one or just not the 50 model? I saw this one on screwfix, only looking at screwfix in particular as I can possibly get it done tomorrow and report back.Read back, I told you what type of pump to get. The old Grundfos are no longer available (50 made redundant, so clearly I won't recommend that make)
https://www.screwfix.com/p/grundfos-ups3-15-50-65-domestic-heating-circulator-230v/503fh
It's labelled as 6.5m head and 15-50/65 so does that look appropriate? Or any specific models you'd recommend?
Main reason I was hoping for a grundfos replacement is that it should apparently be the same size so won't be a problem fitting into existing pipework.