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 (£39.99 now 45% off 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.   

Click on image for type of finish. 

Natural Invisible Matt

Designed not to change the appearance of the slate. Only 1 good coat required.

Also available in Satin finish,

Seals and leaves a surface sheen.

Also available in Gloss.

Ideal for interior and exterior Polished and honed Slate, Apply with brush, cloth, roller or Spray gun. Wash all brushes in soapy water.

covers 8 to 10 sq metres per litre,