Hi, I am an electronics newbie and owing to my own
stupidity I need a new fuse for my multimeter.

Not measuring voltages on the amps range again? ;-)

The manual says get an "identical" fuse, then gives
a colon, and specifies 250V and 500mA. It also says
phi5X20(mm), which I assume just means 5mm in diameter
and 20mm long.

Very easy to find. Buy quickblow rather than surge/delay, unless you find
instructions to the contrary.

Maplins say the only fuse that will do is the one
with their product code N65CA.

********.

Christian.