Basic ingredients are N-P-K
N Nitrogen helps the plant build protein. Plants will take up nitrogen at the expense of other elements. Too much nitrogen will lead to foliage rather than flowers. High nitrogen fertiliser is used for lawns.

P Phosphorous is needed for good root and shoot growth.

K Potassium is used for general vigour. It regulates the movement of water and food within the plant and encourages fruit development.

Plants also need traces of other elements particularly:
magnesium to help make chlorophyll - the 'green stuff' needed to process sunlight.

Calcium builds cell membranes (the equivalent of our bones)

Of the ones you mention:
Tomorite is good for fruit and some veg.
Phostrogen and Growmore are brand names - they should also have a product name such as "all purpose"
Fish, blood and bone is a general balanced fertiliser that does a bit of everything.

Good answer from Urbanite - basically he's saying look at the NPK readout on the packaging. You will notice that Tomorite has an NPK of 4-3-8, (although other makes of tomato feed are higher - I have one that's 6-8-11), whereas Growmore typically has an NPK of 7-7-7. Fish blood and bone is also balanced, usually around 5-5-5. Some feeds will contain other stuff (Toprose for example) - trace elements that are important for different plants. Camellia and rhododendron feed will contain other ingredients which help to acidify the soil, but again, in neutral to acid soils, it's not necessary to buy it.

All this means is that something like Growmore is a good all round general fertiliser, and can be used on most things, in particular around shrubs in the ground. Feeds with higher potassium (K) are, as Urbanite says, used on plants which produce lots of flowers and/or fruit, so are more specialised.

As for specialist rose food, yes, I do think its worth buying something like Toprose, (it also contains iron and magnesium) although you certainly don't have to, and it probably isn't worth it if you've only got one or two roses. As for clematis feed, I can't find any info on the NPK readout, but it does appear to be a slow release formulation. I still wouldn't bother with it though, Growmore will do perfectly well.

Granular formulations usually feed for about 6 weeks, pelleted feeds up to six months, small granule feeds anything between 3-6 weeks, and then there's the liquid type, diluted in water, which are more or less instant, but don't last long in the plant or soil.

Then there's a whole other aspect to the feed question - the best way to 'feed' your plants is, in fact, to improve the soil. Adding humus rich materials to soil increases bio diversity and fertility levels, and enables your plants to take up more nutrients more easily. The type of feeds you're asking about are useful, but aren't a substitute for healthy soil, more an adjunct. Pots and tubs, though, do need these types of feeds.

Thank you for that. So am I right in say that I should spread growmore around my flowering shrubs now and water it in.

My garden is quite densely planted so in the summer there is not much earth between the plants so I have given the flowering ones tomorite occasionally as I thought it promoted flowers.

Is this 'good practice' 'not necessary' of 'overkill'

The reason I am asking this is because just over two years ago I had a new fence and at time I bought six flowering shrubs from a very good local nursery. All the plants are healthy and growing well, but I have yet to see any flowers on the lilac or philadelphus, and the weigelia flowers were quite sparse.

If those have only been in two years, their flowering will start/improve as time goes by and they mature a bit.

Yes you are right about the Growmore round the plants. You don't have to water it in, you can wait for the rain to do it, but if the garden's dry anyway (sounds ridiculous after all the rain we've had, but the top few inches do dry out fairly quickly) then yes, you can water.

I wouldn't bother with the tomorite round shrubs - the 'coding' information for flowering is often formed the year before for some shrubs, and if you don't hit the right week or month when that's happening, your tomorite is simply wasted. You can use it on summer bedding though, specially in pots.

Thank you, I am much clearer about things now.