Antony Gelberg wrote:

Haynes suggests using a drift for this, but surprisingly(!) I don't have
anything to fit that diameter. It looks like something that should be
possible with a bit of direct hammering on alternate sides, but no dice

It is to be expected

so far. Before I ruin the new caps and / or miss my MOT appointment at
12 tomorrow, any advice?

Use a piece of some metallic pipe with matching inner diameter. Or use
the old brake discs which are a bit heavy and awkward to handle, but
luckily have the exact inner diameter you need
Otherwise you have no chance of hitting the cap straight and it will go
askew.