Just ring them up. You pay £25 but this is redeemable against any products you want to buy - so basically the trial is free.
That's if you have it done in store, if you want someone to come out to you then you have to discuss this with them privately. If you just book a trial, the artist will give you her number and you have to call her out of work hours as she's not allowed to make private bookings at work.
As they got my appointment mixed up I had to go to a different store which is why it was a nightmare, I ended up going to a really busy shopping centre. If you went to a quieter store it wouldn't be so bad.
I brought loads of stuff, thinking I would need to top it up as the day went on, but I never did once. It lasted all day and everyone commented on how lovely it looked.
Just be careful because its addictive. I will never buy cheaper makeup again, I've already spent a small fortune in MAC.