DIY cheap tom & other veg feed

I was in wilkinsons the other day and spotted their lines of plant food and then tucked underneath boxes of potash, ammonia etc etc.

Now i was going onto thinking - would a DIY mix of these in a designated watering can be more economical and more nutritious than the cheap stuff I usually buy?

Yes sort of, just read the backs they will have all the relevant data etc.
If your growing leafy crops then a feed mainly with a nitrogen based fert' . Roots crops something like super phosphate. Whether it's a good idea to mix them is another thing you may want to consider feeding the different compounds at different times and perhaps weakening the mix for things that you feel you need less of like say nitrogen for spuds for instance. But I'd not want to make things over strength.
If your organic then avoid using them. Stick to blood fish bone, or similar.