I bought some this morning from Travis Perkins, allegedly for 2" wide
joists, which is what I asked for (the receipt says they are 50mm).
However the actual gap into which the joist fits is only 42mm, which
means I'd have to trim 8mm off the joist width for them to fit.

This is my first time using joist hangers (these are timber- masonry)
so forgive the ignorance; but have I been given the wrong size (which I
suspect) or am I supposed to reduce the joists to fit (which sounds
bizarre)?

I'd rather make a fool of myself asking the question here (where the
regulars are rather more forgiving of muppets) than back in TP...

Thanks
David