Hi Lewy, welcome to the forum.
There is a tremendous skills shortage on the railway at the moment, so even with the Recession looming there is a requirement for technical staff. In this month's copy of Railnews, Network Rail is crying out for new blood.
There is a good source of employers here....
http://www.railwayregister.care4free...S%20&%20DEPOTS
Although that page is called "How to become a Train Driver" it contains useful adressess for all the major employers.
If it really is filthy old locos that you fancy (and I don't mean that disrespectfully to all the FOC men and women out there
), then you should seek out EWS, GBRf, Freightliner (both Heavyhaul & Intermodal), & DRS.
Don't forget that some TOCs also have diesel traction and the occasional loco.
Additionally there is plenty of diesel on-track machinery which needs alot of loving. Colas, Fastline, etc are the people to approach.
Don't be shy, you've gotta to do some legwork. It's perfectly normal to make speculative phone calls in the modern Rail Industry, and sometimes you can just speak to the right person at the time. Get on the phone and find out what's available. If any of them treat you with contempt then you know you don't want to go there !
(Trust me - as a Passenger Train driver trying to get over onto the freight side, I spend half my life making cold calls !!)
Good Luck,
FP