I want to stop my lawn from turning into straw in the hot weather.

There's a threaded water tap in the back wall of my house. I thought
it would be pretty straightforward to put a hosepipe on there, so I
bought a Hozelock 21mm/26.5mm threaded tap connector. I thought the
tap was 21mm, but the connector seems to be slightly too small. The
threads look like they're compatible, but try as I might, I can't
screw the connector on. I've even tried filing its thread down a bit
with a pen-knife, but no joy.

Has anyone else had this problem? Is there a trick to fitting the
connector that I've missed, or do I
have some sort of bizarre non-standard 22mm tap? Would a universal
Hozelock connector solve this problem, or is there something else I
should try?

Any advice much appreciated.