Why is it suddenly leaking around the valve? It wasn’t before.
DT wheel, Stan’s tape, DT valve.
I’ve put a rubber washer under the nut on the valve which has enabled it to seal now, but surely no air should be getting through the tape????

I’ve had this in the past, was nothing to do with the valve. The tape was leaking, tear from a tight tyre fitting, and sealant was escaping from the valve hole. Presume path of least resistance.

put a double thickness of electrical tape round the rim.

valve/tape interface is the weakest link
how old is the installation?

… for 100% reliability, re tape it

What rsvktm said, your tape has probably split somewhere on a spoke hole burr or something.

Whatever you do, don’t be tempted to tighten the external nut too much-I managed to pull a valve into the rim section doing that

Thanks. I’ve opened it up and the tape is fine but the valve has rotated 90 degrees and that is probably the problem (DT valves don’t have a round rubber bit on the inside like Stan’s valves but kind of go along the line of the rim, so if they aren’t in line there could be a gap). Still at a loss as to why it happened 2 hours into a ride though!