For the assessments, use the practise materials until you can do it perfectly. Then set yourself some reasonably basic mental maths questions, so adding, subtracting, multiplying, doing percentages of things etc. Also, pick up a timetable next time you go through a station, and make sure you're confident in how to read it.
Group assessment wise, just show you're able to work as part of a team and you'll be fine. Make sure you don't come across as a mouthy 'un, and don't commit the cardinal sin of forgetting to talk at all. This one, you can't teach really - the best advice is just to make sure you're aware of how you're communicating on the day.
If you get through to the interview stage, it'll be mostly 'Give us an example of a time when...' questions. Have a good range of examples of situations ready - assuming you've done retail and customer service roles before, you'll be fine. Never hurts to throw a few mentions in of good safety practice and such too.
At the end of the day, they want someone who can be professional and maintain the company's image, but has a personality and dedication to the role. And you'll be joining a good team at Bristol, so good luck with it all.
Oh and, before I forget - if it says "Bring a passport-sized photo", make sure you do
