Anhinga sighting in Wiltshire?

I saw a black neck gliding across the water in Swindon. It dived, then popped up again a distance away.  Then it walked out of the lake. I only had a slight glimpse and it looked a similar shape to a heron, but smaller and black. There wasn't any orange on its head so not a grebe. 

I found a photo on Google and can't see that they are usually in the UK, so I'm thinking this might be very rare?