The class 25, although rather plain and unassuming, was a hard working and reliable loco that got just about everywhere on BR and there can be few pre 1987 D&E era layouts that can’t justify at least one on the roster.
Modelling these locos accurately in 4mm scale is sadly not as easy as opening a blue or red box and plonking said loco on the track. Hornby produced a model of the early bodystyle in the mid 70s and although the body is fundamentally the right shape it needs a bit of work to bring it up to date. The chassis and mechanism are typical of the times and are better replaced with something better. Bachmann have produced both the early and late body styles and sadly they are badly compromised by the windscreens leaning back at too much of an angle and the curve above the screen being too shallow. The chassis is also a bit ‘filled in’ and needs work but at least the mechanism is beautifully smooth.
Jim Smith-Wright produced an article in the DEMU Update magazine back in 2007 on modelling the class and how to modify the Bachmann chassis to look more like a real 25 and this article is now available on Jims P4 New Street website http://www.p4newstreet.com/articlePDFs/25%20article.pdf I can recommend anyone looking to improve their class 25 to have a good read of this before starting. More recently Jim has updated his models further and James Wells has put his thoughts on modelling the class on his Eastmoor blog http://eastmoor.blogspot.co.uk/2013/09/project-25.html and on RMWeb http://www.rmweb.co.uk/community/index.php?/topic/68176-an-approach-to-modelling-class-25s-springy-bits/ Also in the intervening years Brassmasters, Shawplan and Penbits have brought out new products so that anyone can built a top drawer 25.
To model a 25 with the early style bodyshell with the grills on the bodyside the easiest way I can see is to use the Hornby body on a modified Bachmann chassis, as recommended by Jim. About five years ago I built a model of 25029 using Jims article and a few ideas of my own. I found I needed to add 1mm to the bottom of the cab and raise the nose by 1.5mm. Since then the general consensus has come about that although the nose may need to be raised a little it is the centre window that needs to be made deeper. I will look into this further if I built another early pattern loco.