Caecilius expressed precisely :
Does anyone know if it's possible to hire RCD testers for short
periods at a reasonable rate? When I've looked, most hire shops direct
me to a PAT tester which isn't what I'm looking for. I don't want to
buy one for a single project.

It would suggest the possibilities of hiring is doubtful, but DIY would
be not really that difficult....

A suitable pot in series with a resistor to limit the maximum current
between the L and E. Then just tweak the pot to the point where it just
trips. Measure the overall resistance, you can check your mains voltage
and from that work out the trip current. All you are missing is the
time, but that might not be so important to your needs.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk