Review by the Guardian.  If you want to do it yourself, go for a top-quality sealer that needs only one coat. Ultimate Brick Sealer from sealer-seal-sealant.co.uk, seals all Brick surfaces. Stephanie Zia. 

This is the first choice sealer of the trade and our best seller - Perfect for all types of Brick.  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.   

1 litre covers 8 to 10 sq metres per litre