That NUJ link shows for a 600x800 image £225 for one month upto £600+ for 6 months, is that really the going rate?
That rate is the suggested "minimum"!
You can also claim interest on these charges from the date that the image was first used.
The Small Claims Courts are very much aware of digital image theft and have handled loads of these cases now.
The reason that the small claims court works so well is that it usually costs the "offender" much more to go to court and defend the claim than it would just to pay up. The majority of Small Claims for this type of thing don't actually make it to court.
It's a very straightforward procedure and unlike normal court procedures the majority of cases favour the victim.
Small Court Charges have recently changed and instead of the fixed charge there is now a "sliding" scale depending on how much you are claiming.
http://www.small-claims-court-support.co.uk/costs.htm
One thing to be aware of though is that you must have tried to claim the appropriate fees in a conventional manner, i.e. by letter or invoice, before referring the matter to the Small Claims Court.
My advice is always to write by Recorded Delivery rather than email and do not under any circumstances telephone.
It all sounds very formal and daunting but at the end of the day you are dealing with quite a high-value theft.
There are no "businesses" that are not aware of copyright theft in this day and age and they are very much aware of lawyers fees and will not want to go to court over a few hundred £'s.
Write your letter, detailing what you think you're owed and refer them to the NUJ charges and see what they say.
billyh42