Since buying the XFR in March, I haven't driven it very much, just a few trips out on a Sunday.

It was on a ramp today, and I've discovered that it's leaking all round the gearbox sump gasket.
The gearbox was serviced back in October 2023. Ive read that it is a good idea to tighten the bolts up a bit more than the specified torque, to avoid this situation.
I'm thinking of getting under and doing just that, and then topping up the fluid.
Does this sound like a reasonable thing to do?

 

That's what I would do. I am no expert on these cars but that's entirely reasonable.

I'd not exceed the torque spec by much, say 10% more. You may find they are less than spec right now, hence the leak.

Also clean well so additional leakage is easy to spot.

 

yep just nipping them up is a very reasonable starting place.

 

Check the transmission sealing sleeve as it's most likely leaking. Pretty much every ZF 6 speed will have this failure. The sealing sleeve is cheap at around $20 but can only be installed with the pan removed.
What happens is the sleeve is mounted up high on the back of the transmission so the oil drips down to the pan and then looks like your leak is there. It's not.

Now this is not bad because with the year of your car it's time to get the fluid changed in the transmission. The filter is integrated into the pan on these units so do replace that and be sure to clean and re-install the magnets in the bottom of the pan. New pan does not come with magnets so don't throw them away with the old pan!

Here is a BMW thread with good picture of what you need to repair.
ZF 6 Speed Sealing Sleeve Leak Repair

Do you know how to top up the fluid? It's not like other transmissions because as you know there is no dip stick?
I don't recommend topping up the fluid again with the age of your car a fluid change is just normal maintenance and now it a great time to do it.
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Jacked her up. After having cleaned the leaking oil, no more oil was evident. And overall it wasn't a big leak at all. None on the undertray.
I nipped all the bolts up. They did appear to be slightly loose. The gear changes are all smooth, so I'm not going to check the oil level just now.
It was serviced just over 6 months ago.
Also replaced the undertray with an intact one and replaced the missing fixings, so it's all secure now.
It had been scraping on the floor. Got one off ebay for £72, which I think is very reasonable.