I had nowt but problems with that site, when trying to book my first jab, it thought I had already had one, but didn't know which vaccine as it wasn't on my vaccination record, and I couldn't get any further. So, I phoned them 3 times, and each time I was promised call backs, that never happened.
The following week I got an invite from the GP & an appointment a couple of days later & second one in June, so instead of driving to one of the NHS centres in Brighton or Chichester, it was a short walk down to the surgery, so all worked out well in the end.
After I booked with the GP, I thought I would check out the website to so if the issue had been resolved, and it told me I had missed my first appointment, just checked again and now it says I've missed my second appointment, bloody hopeless.
Across the coastal strip of West Sussex, there's one booking number for all the GP surgeries & hubs, and I know a couple of people that called them as soon as their age group became eligible, but before getting the GP's invite, and got appointments within 2 days. Certainly here, the GPs seem to be racing ahead in competition with the NHS direct system, there's a lot of money for them to make if they get to people first, as they are getting £12.58 per jab.
So, if you're still having problems, it may be worth checking your GP's website, and see if they have a dedicated booking line, or even just ring the surgery if you can't book with the NHS direct, and you may be lucky.