Devon,
United Kingdom 
Compost has been difficult to access. My milkman stopped selling it for a while. So i tipped all my dried up pots into a container then added bought topsoil and soil conditioner, garden compost, leaf mould and grit.
My plug plants have come on very well; astrantia, echinacea, achillea and fuscshia. It's gratifying to be able to compromise in extremis.
6 May, 2020
Answers
I think we are all brain-washed into thinking that the only thing to use is new compost. Years ago it was sieved soil for seedlings and your own compost for everything else. Let's get back to being a bit more self-sufficient, the plants won't mind and well done Merlin.
7 May, 2020
Thankyou Bathgate and Honeysucklefor your comments. I've learnt that there is no real need to chuck used compost and that it can be useful mixed with other organic mediums. I do love my garden compost and leaf mould and intend to use garden compost more in the future.
9 May, 2020
I agree, there's no point in chucking used compost. It all ends up in the same place. I would add to the compost pile.
10 May, 2020
Previous question
I'm using fallen leaves from last autumn. It makes a very rich compost and brings in lots of earthworms. It's free.
7 May, 2020