Ofcom completed their maintenance on Monday morning and I was able to login and swap my RSGB candidate number for a foundation level call sign.
The website is relatively limited and will only let you search for 3 letter suffixes rather than begins with / contains / starts with etc. In the end I gave up and let them allocate me a random call sign. I ended up with M6IJY (Mike 6 India Juliet Yankee).
I plan on doing some operating at the weekend at my local club, so hopefully I will make my first contact then.