I have one. iZettle are the firm, you just need an Android Phone or an iPhone and download the App, it connects to the reader and off you go. No monthly fee but they charge 2.75% per transaction, this does go down if you have a turnover of more than £2,000 per month on the card.
I have never had any issues with them and would definitely recommend them. If you are interested let me know and if I send you the link you get a £20 voucher.