What fertiliser/feed/etc should I buy?

Slug pellets are a must as you've guessed 

You need a good general fertiliser such as Growmore or Fish Blood and Bone if you're organic.
A handful of either before planting/sowing and then every couple of months during the year, while stuff is growing.

That'll do you fine until you find your way a bit 

The only fertiliser I buy is lime - if you can call it a fertiliser?
and I rely on teas made from horse manure and comfrey to feed the plants
and more manure- dug in during the autumn dig-over and as  a mulch during the growing season. It works for me so far!

Stratts, before you start using manure ....................make sure you have read the advice for testing it for Aminopyralid.
Unfortunately the problem is still around 

Yep have read about the bean test thanks it's on my extensive list of to do's! 

I'll check Slugstoppa too, thanks

Stratts- Manure Tea!
As MoS says- find a clean source of horse manure
Put a bucket or two in a hessian sack and tie is up so the contents won't spill out. Put in a dustbin with a lid and fill with water.
make sure the dustbin is away from the house as it can snell a bit.
Leave it for a couple of weeks or until the liquid inside is discoloured.
Don't worry if there's a bit of a skin or scum on top.
It will go darker the longer you leave it.
Load your watering can with water and put in some of the manure tea until the resulting mixture is like weak tea and use this to feed your plants.
Works a treat!
You can top uo the dustbin with water and repeat until the water inside is itself like weak tea. Then it's time to empty the sack onto the compost heap and refill with more manure.
There are some quite precise ratios published for mixing - 1: 10; 1:20 etc.
I find by mixing the feed that goes on the plants to the colour of weak tea it comes out right without having to worry about mix ratios. Too strong might do some harm(??)

I do something similar with comfrey leaves, but leave them to soak for a good month beofore use.
I believe you can do this with nettles also, but have not tured it yet for myself.

Hope this helps and good luck.

Hobnails.