Hi Barrie - not sure why the experts on this forum haven't replied to you yet, but I'm reading a lot about soil currently, so I thought I'd have a stab at it.
The compost you've already used will be drained of certain nutrients, the balance of which depends on what you've grown in it and how intensively. Although you could theoretically add new nutrients to that compost and try using it again, it would be difficult to determine exactly what is missing (including micronutrients, like magnesium) not to mention the challenge of replenishing it back to its original composition. Then there's the risk of pests that might have laid their eggs to hatch the following year.
For the record, I have used compost from my pots more than once, particularly when the first crop was very quick to harvest and I've got time for a second in the same season. At the end of the year, I dump all used compost onto my growing beds, which are so heavy with clay that I'm convinced adding anything else will help lighten the structure. I assume the pests from the greenhouse couldn't stand a chance in the south Welsh wind and rain.
I've only tried horse manure as an addition to the growing beds, but I've heard that very hungry plants can grow directly in such material (pumpkins, courgettes, etc). If you only have space for containers and there's really nowhere else to put your used compost, I would be tempted to change only half of it each year: 66ltrs sounds like more than any plant could exhaust in one season!