Why have some of the trusses on my tomato plants turned a goldy brown colour whilst they were still very small.

Could be blight. This is caused by damp and warm weather. The rain can splash the fungal spores onto other tomato plants. Are the main stems of the plant starting to go brown or black in places?

No the main stems are very healthy looking green colour

Have other tomatos developed normally? I'm just thinking it might be a variety that produces yellow tomatoes - I've grown those before, they're bright yellow rather than red in colour when ripe.

Yes all the others have developed normally

I can only think of blight then.....more likely if they are outdoors than indoors.

Sounds like blight to me...

Are the ones going brown in very strong sunlight?.