If it just start weeping you may have a perforation on a weld at the vent and need a plumber.
If you vented the radiator prior to the weeping the key may be a little worn
Try a new or different key.
I would isolate the radiator at both ends and Try new vent screw or replace the vent fitting if of appropriate type. or as a strictly temp measure apply a little Fernox LSX when dry.(check system pressure)
Thanks... tried with new key at bleed point and reduced weeping somewhat. Rad is now on and no leak at all. I guess I'll just keep an eye on it. Thanks again.
That could indeed be the case...
Inspected the rad early this morning (when it was off) and it was a bit moist around the bleed point. But it hadn't leaked (I'd left tissue paper directly underneath overnight). After it had been on for a few minutes it was dry around the bleed point.
I guess I'll just keep an eye on it.
Thanks again.