Also, since those maps were done, O2 and Vodafone now do network sharing (i.e. use the same transmitters).
No they don't.
Its not the actual network that we are sharing.
Basically, both us and O2 are currently sharing cell sites rather than network, if we have a mast in an area O2 are able to add a site on that mast to help boost coverage at a cheaper cost than placing a new site in that area and vise versa.
It is a cheaper way to boost network coverage for both networks as to build a new mast can be very costly and timely due to issues with planning permission and cost of building on other peoples land.
http://forum.vodafone.co.uk/topic/58930-information-regarding-o2vodafone-network-sharing-agreement/