Couple of points here:
1. The new rules apply only to Annex I aircraft - those being administered by EASA. Annex II aircraft come under state (UK CAA for us in the UK) control and will continue to be maintained and administered under the old state rules (BCAR etc). To find out if your aircraft is Annex I or II, see this document:
http://www.caa.co.uk/docs/33/CAP747.PDF Section 1 carries the list.
Dragqueen - your C120 is Annex II so your maintenance will be carried out under BCARs by an M3 organisation or similar. You don't need to worry about EASA (lucky you!)
2. A maintenance company can be Subpart G and F approved, which means that the same person may manage your aircraft's airworthiness and also do the work on it when required. You can have the aircraft managed by one company and worked on by another, but I think this will be unlkely in practice. Most companies I know want to keep the whole operation in house as thety do now.