Hi Jensen. If it's been kept closed etc. then no problem. I thought when I saw this that you meant can I re-use it? Well to a point yes - as mulch. But I don't recommend using compost that has been planted up before (I don't think many people will). The nutrients are used and disease is likely. But on what you describe, go ahead.

I have to own up to occasionally re-using multi-purpose compost in summer containers. I remove any weeds, and add plant food granules. It has always worked for me ;-)

yeah me too melchisedec, my dhalia i grow in pots every year(7 last year more this year) and the used compost gets re-bagged after the tubers have been removed then in early march i dig the compost into my veg garden 'job done' ive got some lovely asparagus coming up this year as proof it still works second time around.

Thanks for your replies. Windy64 and Melchisedec - I meant to ask 'can I use compost which was bought the previous year (spring) which has not yet been used'. As I realised from Sarraceniac, this was not very clear - sorry about that.

After posting my question it occurred to me that I might also find answers on websites, and because I want to sow my seeds a.s.a.p. I googled 'multi-purpose compost old'. Your advice as well as other website advice has given me the confidence to use it.
I thought members might be interested to see this piece of advice on a website called chat.allotment:

"If the bag is left outside nutrients could be leached out by rain.
If stored inside soluble nutrients could migrate to one end of the bag so would need to be well mixed before use next year. Could also help to leave open after mixing for a while before use in case there has been a build up of ammonia from breakdown of organic nitrogen".

I must admit that the multi-purpose compost, which is Wickes, 'nutrient rich', 85% peat-based (approved long term after use programmes are in place) is dry; sorry that I did not mention this before. Does this indicate a loss of nutrients? I am wondering if I might add some of my own compost, although it is not three parts 'brown' to one part 'green', but rather the opposite!
I thought that tea leaves, of which there are a lot, counted as 'brown', but a website about compost that I visited puts them in the green category.
My compost does however look good, and so I am thinking of making a mixture of 90% Wickes and 10% of my own. I welcome any advice on this too.

Nutrients leaching out of the bag is a possibility.
Soluble nutrients migrating to one end of the bag is a load of codswallop.
An opened bag stored in the dry will be as good as it was when first opened.
This compost sounds a bit to coarse for sowing seeds.
Sieve a small portion of it, enough for a seed tray and use that.
I would not mix your own compost in with it.

Thanks for that Scrumpygraham. I wish I had had the patience to wait until getting your reply.
The Wickes compost does not look coarse, but I sieved it for my next pot of seeds anyway, and it feels lovely.
I also took your advice not to mix it with my own compost.
In fact, I have a question about my own.
Several days ago, I took some straight from the bottom of my dalek compost bin, and sieved it. However, there was quite a bit of life in it, apart from the worms and some ants which I removed. I planted some seeds in it anyway. Is this alright, or will the life eat the tomato seeds?

It sounds as if your compost has a bit more "composting" to go. Worms, will make the compost better.
I would never use compost from a compost bin for sowing seeds regardless of any "life in it", mainly because it would be better used elsewhere. Provided you get the correct temperature then your tomatoes should germinate.
Only one way to find out.