I looked at a lot of grammar books - including books published in Spain - before buying this book. This is by a long way the most accessible I have seen (although I must admit I haven't I haven't compared it to the Butt/Benjamin New Reference Grammar).
The book has two parts - the first is a concise traditional-style reference grammar. This is well-organised and clear. It is intended to be a reference grammar but the authors go out of their way to explain grammatical concepts in accessible terms: for example the opening chapter is a glossary that explains things like what an adjective is, what an auxiliary verb is etc. the explanations are clear and accompanied by lots of examples. It certainly doesn't assume that you have a GCSE.
The second part (which takes up over half of the book) is organised around specific activities and tasks eg 'asking and giving opinions', 'expressing agreement and disagreement', expressing desires and preferences'. Again clear explanations and lots of examples.
It is a reference grammar. It is intended to be consulted as needed. If you want to learn Spanish grammar then there are other books such as the 'Uso de la gramatica espagnol' series or the 'Paso a paso' series - and their English-language equivalents. I'm very pleased with my purchase - I think it will prove to be a great investment and I expect to be using it for a long time to come.