Just a suggestion (I'm no tomato expert) - you could try and cut off the shoots that haven't got any trusses. That would give more light and ventilation to the ones that have.
Also take out any new side shoots from now on.
I don't know if that's the right thing to do, but it's what I might try if it was mine.

I agree with Hywel, remove the non-fruiting stems and pinch out any new side shoots as they appear.

You do need to pinch out the side shoots as this is taking all the goodness from the tomato producing fruit. I take it the tomato is only indoors for the photo's sake - and obviously should be growing outside. Apart from that it looks very healthy and needs feeding once a week but I assume you already do that.

Yes, the plant is normally in a greenhouse and I am feeding it once a week.
I have been pinching out all the new side shoot growth for the last two weeks, since realising that this plant is not a bushy/tumbler type.
There are eight yellow trusses - most of these stand out in the photo, and there are as many as that again, less developed. I have four plants like this, and I have noticed that on the bottom two branches on each one, the trusses are either a lot or a bit less developed than on the other branches.
If I remove any shoots/stems/branches, is there a risk of letting in disease and mould?

No they should be ok - I am still pinching out shoots all the time, because there is usually one hiding in amongst the leaves. My plants are grown in bottomless pots on the soil in the greenhouse (I think its called ring culture) and they now stand about 5ft tall.

Each of these tomato plants has about ten diagonal branches, pointing outwards and upwards - the first four at the bottom, and then the developed side shoots above, which were not pinched out. These side shoots have dominated the leaf-only branches that grew out before them (the ones that you leave), pushing them downwards. The four bottom branches and the side shoots have all produced trusses, and I am wondering which (if) I should remove. It feels a bit painful removing them, but of course the plants will only last until September because of the weather. Any advice on this?

I'd remove all side shoots that do not have flowers or fruit trusses. You are trying to grow tomatoes not produce a pretty bush.

Thanks Moon growe. The thing is that all of the side shoots have flowers/fruit trusses, though some less developed than others. This makes me hesitate about removing any of them. Perhaps I should take out a few of the branches (these are not side shoots) lower down, with less developed trusses on them?

We tend to only allow 4 or 5 trusses of fruit to set and then pinch out ruthlessly - particularly when the tomato plant is in a pot there is a limit to the amount of fruit it can grow to a good size.

That is a good point you made regarding the pot Moon growe. I think I will start by removing some of the lowest branches (as opposed to side shoots), as they are growing outwards and really making the plants spread out. The one on the far right in the photo is now horizontal! and there is one like this on one of the other plants too.
Do you think this is alright?

I see now what you mean about removing the non-fruiting stems. I have also removed some of the side shoots.