The cheapest wildcard certificate I can find is £99 
  per annum.

Best I can find is [1] which is $61=£36/year, or [2] which is $42=£25/year but with lots of typos on their web page.

[1] https://cheapsslsecurity.com/comodo/positivessl-wildcard.htm... [2] https://www.ssl2buy.com/cheap-wildcard-ssl-certificates-ads