Right I've read up a bit more and have come up with the below plan:
Aim for under 50g carb a day on fast days, and under 75g on feed days (but up to 100g is fine). At the weekend if the opportunity comes up for cake/pasta I won't pass it up, but neither will I seek it out. I think it's important for me to not feel like I have a set of hard and fast rules as I will tend to beat myself up for breaking them, and then give up! I tend to plan meals and shop around that, so I should only have to deviate if I'm out for dinner. As long as I stick to properly low-carbing for 5 out of 7 days I'm hoping I see/feel a difference. This plan will involve tracking what I eat on feed days, which kind of goes against 5:2, but I don't mind too much. Hopefully after a few weeks I will have learnt which foods are okay and I won't need to track as much.
I'm going to try and eat fruit for puds - I know some are carby but if I don't have something sweet after a meal I just sit and think about that fact for the rest of the evening. And then put on my Uggs and go to Tesco and raid the bakery shelf (unfortunately we live 30 seconds from a Tesco Express and this is all too easy to do).
What is going to be hard to get my head around is that I can have cream on my fruit, olive oil in my cooking and even butter on my (celeriac) mash. One thing I am REALLY pleased about is cheese. I love a good extra mature cheddar, but for years (WW) I've had to ration it and then every now and then would succumb to a late night craving and stand at the counter eating slice after slice! Now I can just incorporate it into my meals

I am mostly worried about work lunches, we have lots of options but all geared towards people who eat 'normally'. The 'salad' bar is great but unfortunately I only really like sweetcorn and carrots (both carby) from the veg section - the rest is all cous cous/pasta/rice salads. Can't stand lettuce which narrows my salad options somewhat. We have a Waitrose but again, it's all sandwiches, wraps and pasta salads. Will have to think about that! Actually while writing that I remembered about the 'grill' with grilled chicken, beef etc. Hopefully I can have that option without the carb!
I'm a bit nervous about what's ahead, and really hope I last more than one day.
Today's dinner (fasting) is tandoori chicken in a marinade I made last night with full fat Greek yoghurt (yup) and a tomato, red onion and coriander salad. Rice for DBF!
Feeling determined and looking forward to seeing results before my holiday (where I will be eating baguettes for a week).