So I got a reply from the Great North Swim website and their advice was as follows:

Dear Paul

If you have never done open water swimming before I would start with the 1 mile event and see how you go and then next year you could move up to the two mile event

Hope this helps

Kind Regards
Customer Service Team

Without being too hard on myself I think they are right. I’ve never done ANY open water swimming before and it’s a huge learning curve doing 2 miles straight. So I’ll go for the 1 mile event then. My friend Hellen still reckons I should do it front crawl due to tidal movements as it is faster than breaststroke. Maybe that could be my weight, the bit I tell those sponsoring me “doing it breaststroke is miles harder”… hmmmm