You need to update your email signatures
Whenever we tell this to our new customers without fail 90% didn’t know about this companies house law introduced on the 1st January 2012!!
“At certain times it is now a legal requirement to have your Companies Registered Office Address as well as the Company Registration Number with your emails (aside from other media)”
See the above screenshot from Companies house showing this highlighted in yellow. The URL of this site is http://www.companieshouse.gov.uk/promotional/busStationery.shtml
Go and get compliant and change your signature on your emails today.