In my opinion, those who offer money back, or free if you're not happy are trying to hide their shoddy work. LLU spam their name and cold call to get clients then give this "unbeatable offer". Even if they can't fulfil what they say they will, they already have had £300 for doing zilch. Most SEO's will work from word of mouth.
An SEO will link build to your site (without spamming) and will make amendments to your current site. They will increase your ROI and convert visitors into sales, where LLU just send get you onto the top of google by spamming links (from what I have read).
You're looking at this the wrong way, ignore the costs but look at the end goal. Lets say LLU get you £600 of business by you being on top of Mr G for one keyword, while an SEO gets you £1000 by converting current visitors into sales, and getting your site as the same keyword as LLU.
Which would you use? Getting onto the top of Google isn't just about getting links to your site, you need to convert the visitors.
LLU suck, google and UKBF members will tell you this. Use an SEO that has experience in your sector. You will pay more, but you will get a better service.