... If it were me I would remove some of the compost and replace it in the spring with fresh compost. When you have paid only £10 for 3 bags you must have noticed that the compost was full of twigs and stuff you dont associate with compost. This year I had to sieve the stuff I bought before I used it. The increase of bulking agent was dramatic compared to previous years... same old story you get what you pay for.
This year I've had around 80 tomato plants in various locations including my greenhouse, polytunnel, open ground and in containers hanging on a fence. By far my best producers were the containers. They were filled with the more expensive compost that came from Miraclegrow Growbags.
It was about £5 a bag, but worth every penny. I would suggest spending a bit more and getting just a few of these to replace some of your old compost in the containers. If you think about it .. to grow a plant you remove nutrients from the compost. Container plants cant replace these nutrients by themselves, so topping them up is the only solution.
But I would suggest creating a compost bin and replacing everything every couple of years, so as to keep the risk of disease down. If all your garden waste goes in the bin then within a year you will be creating your own compost, which is by far the best, as you know what has gone into it.
Only yesterday I was given another composting cone, I now have 6 of them sited around my allotment ... just load them up and turn the contents with a fork every now and then. Next spring I will have plenty fresh compost without the rubbish in it .. all free !
Ps ... still picking tomato's now ..