Review by the Guardian.  If you want to do it yourself, go for a top-quality sealer that needs only one coat. Ultimate Terracotta Sealer (£39.99 now 25% off from sealer-seal-sealant.co.uk, seals all Terracotta surfaces. Stephanie Zia.  

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

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

1 litre covers 8 to 10 sq metres per litre