Hi Paul,
I get quite a lot of business from Google Adwords. I have done my own since 2004 so have become an expert in them.
In Google you are ranking well in natural listings and your Google Map listing is top position for key words 'carpet cleaning chesterfield'. So really you may not need to do them. It is a view taken though that a top position in organic listings and also having sponsored listings will greatly improve your business. There is a huge increase in the amount of click throughs by being just that closer to the top.
The coupon you have received is a way that Google gets you to spend money with them. You'll also find one in the FSB's monthly magazine, or in computer magazines such as PC Pro or .Net.
If you do decide to do them yourself first you need to read this book by Perry Marshall - The Ultimate Guide to Google Adwords - available on Amazon.
Test the results.
Also I have noticed that you have a Transit Connect. I had two of these for my business, both bought new, and both of them blew their engines. One blew it's engine at 67,000 miles and again at 120,000 (with help of Kwik Fit oil filter failing at 12,200 miles - they said that they weren't responsible as parts warranty is for 12,000 miles) and the other one at 65,000. I fixed the newer van of the two and scraped the other. So I'd recommend getting it serviced every 10,000 miles rather than the recommended 15,000.
Hope this helps
Graham