Ok a basic summation.
Roof's stayed the same through out all locos careers, oblong engine exhaust openings at No.2 end and boiler vent at No.1 end, some variations in boiler vent, some are circular grills, quite large, about 50% of main radiator grill diameter, others have a blanking circular plate over this with an oval hole and small rim chimney sort of affair.
Water hatches and steps are blanked off adhoc from the late 70's, most locos had steps blanked off before the hatch but some at the same time, by the end of the 80's most look to have had all blanked off but you will need to check prototype photos of your chosen loco. The earliest photo I can find of steps blanked and water hatch remaining is 31 146 at York in 78 but I've only had a quick flick through a few books, and 31 287 in Tyneside with all openings blanked off in 1977.
If your modelling later periods then buffer skirts will be your biggest problem, these started to go from the mid 80's but again some members lingered on with skirts for many years, 31 281 in BR blue in 88, 31 304 railfreight grey with full yellow cabs in 87. Skirts all seem to have gone by 1990 and latest image I can find is 31 152 with skirts actually being cut up in 1990.
Many locos were also fitted with radiator fan cowls, again seems a random fitting so check with photos.
Original engines were Mirrlees but from 1964 onwards swapped out for EE blocks, roof stayed the same and Mirrlees locos became class 30 but none carried 30 xxx numbers.
31/4 were taken from 31/1 fleet and a couple (one-two?) were from the early skinhead variety, conversion began in 70 and ended in 85.
So your basically looking at late 70's till the end of the 80's with what you have, with luck an early 90's machine but they'll be thin on the ground to match what you have.
If you want class 31 images then go here
Flickr 31 over 5000 to view and most are taken by serious enthusiasts so will have dates and train information etc, most are numbered so if you want to find a specific loco you just search for that number, most people do not leave a space so 31 008 becomes 31008, that'll get you more hits.
If your just after ETH versions then here
ETH 31 over 700 to view.
In addition Brian Daniels has some nice Flickr collections here for
modern (90 images) versions and here for
historical (over 500) and
walkaround (62 images).
If that's not enough I have over 700 collected myself off the web over the past 10 years or so.
Hope that helps