What everyone misses is that Michael Mosley's 5:2 'diet' was more about the possible health benefits of a restricted calorie regime and how to gain that benefit without becoming some kind of hermit. Weight loss was a secondary concern - more of a side benefit if it happens, and it is not certain it will for everyone.
If it works for you weightwise, the HUGE advantage over Hairy Delias, or virtually any other system, is that once you sort out what you will eat on the two low calorie days the other 5 are just ... eat normally (without going totally silly of course). Pizza after the cinema? No problem, no need to count the calories and plot it or try and rework your 'diet' recipes to account for it. Over Christmas I just moved my LC days a bit so I could eat fully on 25 & 26 as we had relatives staying. I put about 6 lbs on over Xmas and New Year, but despite eating leftover fare (mainly chocolate) since then my weight has actually now almost got back to pre-Xmas levels again.
The downer is feeling a bit hungry on the LC days (today is one of them), but I don't think there are any slimming type diets that work without a substantial chunk of self-discipline. At least it's only 2 days a week