Thanks for replies so far. To answer some of the questions you've asked:
They've been OEC radiators, not aluminium after-market.
They tend to fail in the same sort of area- so I'll see of there's something that is snagging.
The garage I'm using does have a
pressure test kit- I know they checked last time and said it was not overpressured. But let's say it's high on my list of questions this time!
I'll get them to check the rad cap and also any blockages or collapses in the rad pipes.
Thanks for the help- please feel free to add more.