Fitted my new sump today, used RTV silicone sealant to seal it as there is no gasket as such for the sump.
Question is, can I use the car right after fitting, or should I leave the car for 24 hours or so for the silicone to cure?

I haven't refilled the oil yet, I'm gonna do it in the morning, but I just wondered what the general concensus is here as I'd be goofed if I had to use the car straight away!

I drove mine almost right away, an no issues nearly a year on!

I'd say you should be fine.

Curing time for RTV is 24 hours per 2mm depth, so I think filling the oil tomorrow is the best plan. I would hope that a new sump once tightened with a bit of RTV shouldn't leak if you had to drive soon after fitment, but unsure whether the heat generated by the engine would speed up curing time.
How were the original sumps sealed Neil as you mentioned they don't have a gasket?

Sealant used originally too, but its not a silicone sealant, it actually looks like the stuff thats used to bond the headlight glass to the unit. Its grey, non-setting and matt.

just looked on the rtv webside and it says if using rtv on a 323 estate you cant drive for at least a week..

only joking... as long as the sealant is set in the morning you will be good to go.. might just take a bit longer cus its cold out

it's silicone sealant, just the high quality kind from dow corning (and it's bloody expensive... I have 2 tubes sitting here)... the stuff you normally buy at the shops aren't as good but will do the job
as long as you used the grey type silicone, you will be ok