Thanks IJWS15,
I've taken your advice and looked at the info published by Transo. This is taken from here:
http://www.nationalgrid.com/NR/rdonlyres/7DC3282C-33DE-46A6-B697-152DEFECBA85/1118/3eb1.doc
3.7 Shallow Depth Services
3.7.1 The service pipe should normally be laid at a minimum depth of 375mm in private ground and 450mm in public highway. Should a situation where the service pipe is not at the required depth or proposed to be laid shallower1, then consideration should be given to either additional protection2 or replacement to minimise 3rd party damage.
Notes:
1 Proposals for shallow depth services will need to be validated by Transco and recorded by UIP on completion of the work.
2Examples of additional protection against interference would include the provision of a sleeved duct, concrete plinth, steel plate with a suitable caution / warning tape applied.
The HSE website FAQ says:
http://www.hse.gov.uk/pipelines/faqs.htm
HSE expects mains and services to be laid at the depths specified in these publications unless other effective precautions are taken to minimise the risk of third party damage.
A gas main should normally be laid with a minimum depth of cover of 750 mm in a road or verge and 600 mm in a footpath.
A gas service pipe should normally be laid with a minimum depth of cover of 375 mm in private ground and 450 mm in footpaths and highways.
so it seems it depends where it is?
Our existing gas supply is exposed on our drive at ground level and was literally underneath the tarmac. Think they were having an off day when that was laid.