Talking of class 56s, just seen 56312 out working between Hams Hall and Dollands Moor for the first time in months.
What colour was the 56 you say you saw? As there are only a handful of mainline registered ones, it might be possible to identify it by livery.
56301 to 303 are in Fast line grey with yellow and white stripes,
click for example ,
56311 is a pale grey with yellow cabs
see here and 56312 is an unmistakable purple with bright green writing
like this . I seem to recall that there was also one turned out in a dark green similar to GW livery, but am unsure if it ever got as far as the mainline.