I had one permanently set aside a few years back, having moved address and the court claim was delivered to the wrong address. I have also heard of set asides for people who have been out of the country, ill etc. Essentially, the law will allows set aside if the defendant can demonstate that they didnt have or couldn't have had received service of papers..
Its a very simple process, fill in the court form for set asides, pay the fee and the court in time will give a date when they will look at it. I had an informal 5 minute hearing on my one, the claimant didnt turn up/or write in to defend and it was set aside. End of.
Bear in mind though, once the Claimant is served of your application to set aside, they may well start the process from the beginning, but I guess at least it should be alot less cash to find!