Robbie, you might be looking at a becket block (aka "block with becket") or
fiddle block, or something else. If you mean a name for the entire set of blocks and all--no, I think we all just refer to that as "all the stuff on the boom".<G>
But you can get some better ideas by looking at
Harken Yacht Equipment and selecting the UK or other link,
Harken shows a lot on their web sites. Here in the US, they are also very responsive to emails (I'd expect the UK shop to be the same) asking about what you have, what you need, and they very much understand "I've got a...thingy...what do you call it?" <G> Along with suggesting alternatives.