If you know that the compost is of poor quality I can't understand why you would buy it in the first place? There are lots of reputable dealers that sell good quality compost so why not ask around and deal with them?

If you really need to add something to give a fertiliser a boost, then I would add a general slow release fertiliser such as Growmore.

There was another question recently about compost, put it in the search box top right formore info

I add slow release capsules when potting plants up. The fertiliser in most composts is only good for a few weeks in any case.

None at all at this time of year. Most come with some fertiliser added, which should last about 4-6 weeks, after which you can feed whatever's growing in them - but only up till end of June if they're permanent planting, or till the end of the season if they're temporary, like summer bedding.

I add slow release capsules too so that when the stuff in the compost is finished then the capsules do their stuff.

I do too, and in my tomato grobags then use tomorite as normal, seems to work, I'm still getting new tomatoes

I also find it's worthwhile using slow release granules as a boost even though it's getting late in the season and the plants will need less feed as it becomes cooler. Better than a conventional fertiliser which will feed in a rush and also produce soft growth which can be burnt by frosts.

Thanks to all for advise on composts.
Someone told me to try "Verve" from B&Q I found it better than some I'd bought, I mixed it with some JI no 2 and a few capsules,as advised, for potting up and it worked a treat.