Hi Chubbs. Realtime trains is the place to look:
http://www.realtimetrains.co.uk/search/advanced
Set your date, times & locations and probably change 'all services' to 'non-passenger services' to filter it out a bit more - then submit.
From the resulting list, you should look for anything in the ID coumn which reads 'FRGT' (in light blue). These are generally freight or engineering trains or light engine movements, but beware - some of these are listed as 'runs as required' and so may not run every time that they are listed. Clicking on the 'FRGT' gives you the full itinery for the trip and at the top of this page it may say something like 'runs as required - this message will disappear if the train is activated in the working timetable'. Special trains such as steam or diesel hauled charters are also listed as 'FRGT'
Down here in Sussex, there are a number of light engine movements to & from St Leonards depot (Bridge Way, St Leonards) - usually some movements every day.
http://www.railwayforum.net/gallery/...&searchid=9791
The other regular freight in the area is Gypsum trains to & from Mountfield sidings (south of Robertsbridge).
http://www.railwayforum.net/gallery/...&searchid=9792
There are also Rail Head Treatment Trains (RHTT) all over the network at this time of year - some of these are diesel hauled and some use the Network Rail Multi Purpose vehicles (MPVs).
http://www.railwayforum.net/gallery/...imageuser=2380
http://www.railwayforum.net/gallery/...imageuser=2380
GBRf also use Tonbridge West Yard as a stabling point and, although a little further away from Brighton, Ashford sees quite a lot of freight & light engine movements passing through on the way to or from Dollands Moor (for cross channel freight through the channel tunnel).
Also, have you seen that the Bluebell Railway has 2 class 55 'Deltics' visiting for a diesel gala in April.
Hope you can find something to see & welcome to the forum.
Tony