Sweet peas in a container - re using compost

I have this blue pot that I’ve realised fits this frame perfectly so I thought about growing sweet peas in here rather than in the ground.

I can’t remember what was in this pot previously (I don’t think anything last year, so the compost is at least a couple of years old) but rather than waste the compost that’s in it, can I maybe replace half of it with fresh compost and a good couple of handfuls of blood fish and bone?

 

No problem re suing it for s. peas. I'd use a slow release granular feed though, rather than F,B&B which will be more useful. I also give tomato feeds to mine later on in the year.
They're greedy plants, but make sure the compost you add is a soil based one, as it'll retain moisture, especially once the foliage starts to cover the pot and rain doesn't get in well enough. They need lots of moisture in containers, so don't let them get dried out.

 

I have a wheelbarrow full of garden soil, would it be worth mixing that in as well then?

Noted regarding the feed, I’ll go and pick something up

 

Garden soil would be fine. All my pots are a mix of old used compost mixed with home-made compost and/or garden soil

 

Yes - the soil will be fine unless it's light and sandy.
Ideally, for using pots - a good base of old turf or garden soil, with some rotted manure and decent quality compost as the mix, is ideal for s. peas, with some added slow release food. That give them a good start, and keeps them happy for several months. You can also add more compost as the levels drop, which is common through the growing season.
I'd never use BF&B for that sort of planting, especially in a pot.