Stupidly I forgot to turn off a garden tap for a few hours and left a hose
sprinkler plugged in. The back pressure has damaged something in the tap
because I now can't stop the water flowing from the tap.

The water is flowing from the spout with more than a drip - the flow is a
constant stream.

Before I start making the problem worse I'm wondering whether or not the
problem is likely to be with the washer or with the gland. If water is
flowing from the spout is the problem more likely to be washer or gland?

I'm also wondering if it is simpler to just buy a new tap from Wickes and
screw it on to the existing fitting and use some thread tape.