I don't know too much about CATS, in fact I'm a bit worried about the quality of the brush-up, and that's why I'm asking you to share your direct experiences with them and reassure me about their quality. Thanks in advance for your further response.
I'm doing DL with CATS. Actually as I type I should be revising as have exams next week
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I'm pretty happy with them. The distance learning really comes down to the effort you put in to learn the subjects so I wouldn't be too worried about the brushup. It is just the icing on the cake and will not make or break you.
The main advantage of CATS (other than the price!) is that they are extremely accomodating and flexible. They split the exams into three stages, but have no problem with you swapping the order of the stages or indeed picking and choosing exams to make your own stages and using more than three sittings, if you so desire. I'd have no problem recommending them.
That said if you have started with Bristol you really don't want to pay twice! I would speak to BGS in the first instance and try and sort something out. Hard to see why they would object to you attending a brushup and then doing the exams over two sittings rather one.