Does anyone know if when one has a lengthy layover at an airport between legs on their journey (something like 20 hours) whether one is able to leave the airport or whether one has to stay there for the whole time period.

Depends on your luggage situation. Hand luggage only you can take it with you but if you have a big suitcase then you have to be with it at all times when within the airport. There's no left luggage lockers anymore and check-in is not usually open until a few hours before departure.

Al

It depends on where you are laying over

Some placed DO have left luggage lockers
If your bags are cecked all the way though from start to desitination (esp with the same airline) then you may nit need to collect them

Also if you need a visa for the country then you will need a visa

Singapore Airport provides this with a special coach tour and they will look after luggage and no need to through passport control as so long as you stay on the coarch you dont "Technically" eneter the country

So

Where are you going fromk - to and where are you stopping and is it all via 1 airline?

It's now looking like being less of an issue.

I'll be going to Lublin in Poland in June for a friend's wedding and some of the cheaper flight options 'featured' anything up to 12 hours in another vity (Vienna, Brussels etc.) which is what I was considering, but we've now found cheap direct flights which is far more sensible.

Depends. Airlines at their hubs (eg BA at Heathrow) often allow bags to checked in 24 hours in advance. Not always though.