Review by the Guardian.  If you want to do it yourself, go for a top-quality sealer that needs only one coat. Ultimate Slate Sealer (Was £39.99 a litre now only  £24.99, A five litre was £129.99 now only £79.99 from sealer-seal-sealant.co.uk, seals all Slate surfaces. Stephanie Zia. 

This is the first choice sealer of the trade and our best seller - Perfect for all types of POLISHED AND NON POLISHED natural and man-made slate.  You will be amazed at how good this sealer performs with just 1 coat..

Benefits of Ultimate sealer. Repels all liquids, prevents frost damage, protects against staining, non slip finish, leaves a natural finish, No harmful smells or residue are left. Ultimate Matt Lasts up to 15 years, Important - Only 1 good coat required. 

covers 8 to 10 sq metres per litre