Most functions are one-off's, so you want to make sure you get it right. These tips are in no particular order of importance, but all should be considered.
First, before you even contact a mobile disco company, it's vital that you don't have your mind on hiring the cheapest you can find. As in most things, you generally get what you pay for. Especially with a one off event like a wedding, don't you want the very best you can possibly afford?
Let's face it, in a week or two, most of your guests will remember what food they had, what the flowers looked like, how great the cake looked or how the room was decorated... But ask them if they had fun, and I'll bet they'll remember, and who do you think carries the can for that? Damn right, the DJ.
If you're paying £300 plus for the wedding cake, god knows how much on flowers etc., why would you expect to pay less than a few hundred quid for a professional DJ?
P.A.T. stands for portable appliance test and any business using portable electrical equipment needs to have all gear tested. Again this is a legal requirement and they should carry certificates proving all P.A.T. documentation is up to date.
Ok, this one might seem obvious, but make sure the company you're dealing with confirms everything in writing and that they have a written Booking Agreement for you to sign. The Booking Agreement should lay out all details of the function - date, venue, times, overtime rates etc. This Agreement is to protect both the company and the customer and should not be heavily biased in any one party's favour.