I have a large thorn in a tubeless tyre that is leaking air mainly because I had neglected to keep it topped up with fresh sealant. Can I just stick new sealant in and let it seal the hole after removing the thorn or do I need to patch the tyre? If the latter will an ordinary puncture patch do it? The tyre is fairly new and I am reuctant to junk it yet.

more fluid should do it

I had this happen on my second ever tubeless outing, although I had plenty of latex in the tyre a lot ended up all over the rear of the bike, then I had a 3 weeks saga of it sealing and leaking, so I eventually patched the inside of the tyre with a patch, a tip, don't use normal tube glue it doesn't work well on tyres, I used superglue.

some tyres are rubberised inside, you can get special glue & patches otherwise, or use the panaracer one.

I used a standard patch with the glue & once dried I put a layer of mastic over it to seal it completely. Not an issue since.

More sealant just produced a spray through the hole so it looks like a patch is in order!

I leave the thorns in – helps stem the flow.

I've found at higher pressures on larger punctures that the latex just pee's out till the latex can resist the lower air pressure that results from the air loss.

Generally I rock the puncture in and out of the latex – if it spurts, roll it out again, then give it a try 5 or ten seconds later. Keep doing that till it stops spurting and you won't lose that much latex or get it over you and the bike.