Jim asks: Can an acceptable paint job be done on a loco by brush - I'm thinking smallish, specifically a Hornby Sentinel - if so, what's the best way to go about it please?
I think it partly depends on what you term "acceptable". If the answer is, "looks like something Iain Rathbone would do" then the answer is no. There's a good reason he uses an airbrush and a bucket of skill.
If you are after a "layout loco" that will appear fine from normal viewing distances then the answer is "probably".
Most of my locos have the main colour applied with an airbrush and the rest, usually black bits and buffer beams, painted by hand. I do this because I'm too lazy to mask these parts up properly and even if I did, sorting out the inevitable paint creep will take longer than doing the job by hand. Some paints seem to work very well when applied by hand and luckily, Humbrol 33 (Matt black) is one of those.
In pre-airbrush days, I'd have painted the lot and been perfectly content with it. The key is to use new paint and a very clean brush. Work logically around the model so you don't need to go over parts twice - once the paint starts to dry you can tear the surface by dragging a brush over it again leaving marks.
Don't try and cover with one coat. Better to apply 2 or 3 thin coats (and you may need to thin the paint, UK produced Humbrol seems very thick these days) than a single sludgy, detail-hiding one. Large flat areas might benefit from a very, very fine sanding between coats. Brush dust away and store the drying model in a warm and sealed box.
Then consider weathering the model. A bit of dirt hides a multitude of sins. A wash of brown and perhaps some dry-brushing brings the detail alive and unless the main colours have applied really badly, will make it looks like the model has seen service.
One issue will be with transfers. These really do like a flat surface and preferably a glossy one. You can hand paint colour but varnish is another matter. I really do suggest getting a spray can for this unless it's a very small area.
Talking of spray cans, don't forget these. If it's a black loco then Halfords sell a nice selection of cans. They also do a huge range of shades for your main body colour. Of course you could buy modelling paints in aerosol too but perhaps these aren't so easily available. Personally, I haven't painted a black loco black since Humbrol introduced their mini-sprays back in the 1980s (I think) as the results were so quick and good it wasn't worth the effort.
Anyway, the answer is yes. Perhaps practise on a cheap second hand model first, but go for it.