Sundial at Chirk Castle

The sundial at Chirk Castle is in the beautiful rose garden. It is not adjusted for British Summer Time so read 1.13pm. So that's 22 minutes out. I don't think any of the correction curves I have seen are out by that much. I was thinking of taking a compass to check for south but that's no use because it would give magnetic south not the direction of the Sun when it is highest in the sky...