Either that's a very well-paid job or you've missed out the words 'pro rata' from the information.
I'll start by working on the basis that it's not a 'pro rata' rate. (i.e. I'll assume that the information in your question is complete):
At the lower end of the scale, the pay works out at about �16.83 per hour. At the top end of the scale, it works out at around �18.60 per hour.
As I've suggested, those figures sound very high. I think that the rates should be 'pro rata'. The problem is "pro rata to what?". It's impossible to do the calculation without knowing how many hours per week full time staff work but here are a few examples:
If full-time staff work 40 hours per week, the hourly rate will be �7.78 to �8.60.
If full-time staff work 37 hours per week, the hourly rate will be �8.41 to �9.30.
If full-time staff work 35 hours per week, the hourly rate will be �8.89 to �9.83.
So, there are a variety of possible answers to your question. My guess, however, is that you need to look at one of the 'pro rata' figures, rather than the first ones I gave. A further guess is that 18.5 hours means half a working week. So, that means that the 'pro rata' figures for a 37 hour week are the ones you want.
Chris