Guyblo,
Changing industries is never easy, but is possible if approached the right way and not expecting quick solutions.
First you need to have some idea of what it is you would like to do - "engineering within the oil/gas sector" covers a huge range of work. Do you mean design work? In which case you need to figure out a discipline as most of the big design houses in London (Worley Parsons, Foster wheeler, KBR, Fluor, Bechtel etc) etc work on a pretty tight discipline division of piping, mechanical (valves etc), rotating machinery, structures, civil etc.
consultant engineering companies are a bit more flexible, but the lack of relevant experience make sit difficult to sell you so you would need to drop a couple of grades and arrange for a training period before more rapid promotion to mathch your experience in project management. in the last few years the design engineering industry has seen some entrants form other sources. I recently saw a maintenance engineer through a period of acclimatisation and it took about two years before he stopped making silly mistakes due to lack of experience and has finally turned into a very useful pipeline engineer / mechanical engineer.
The route zdas04 suggests is more akin to a oil company operations engineer and work much more at the pointy end of the oil and gas working envelope.
You probably need to talk to a few HR depts. in the bigger design houses or the London consultancies and look closely at your skills and experience in the realms of piping design, mechanical design (valves, piping components), rotating machinery etc if this is your desire, plus ability to work to deadlines, project manage contractors etc and see how it goes. Just don't expect to earn what you're currently on for a while, but the ride is worth it.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way