I have set up a map and use tiles I have previously downloaded. However it appears no extra tiles are downloaded to my hard drive.

To test, I also downloaded MapQuestOSM from http://marble.kde.org/maps-4.5.php, but still no maps are downloaded onto my hard drive. I can see the maps in Marble as they are downloaded only to display on screen, but not saved to the hard drive. So for this test I only ever have folder '0' and nothing extra is created.

Any help?

Where are you looking for these "Downloaded maps" exactly?
If I recall, they are stored at a different location compared to the "cache".

Try looking at ~/.kde4/share/apps/marble.

I'm looking in marble/data/maps - where I installed marble (windows installation). This is where I've put my map tiles (in /earth), and marble is reading these tiles when I'm offline just fine. But any tiles that aren't here it downloads and display (as it should, when I'm online), but it doesn't save any here. Am I expecting the wrong thing from Marble - does it not download and save tiles for offline viewing?

It will save downloaded tiles for offline viewing if I recall, however they will be stored at a different location. I would suggest searching %USER%/AppData for "marble" as a starting point.

That's great, thanks Ben.
I'm a windows user , so if anyone else is interested on windows cache is at "%APPDATA%\.marble"
I can't see why it doesn't save it in marbleDataPath, rather than caching it. But now at least I can use the downloaded tiles.

I believe this is because marbleDataPath is usually in a system wide location, which is often not writable by normal users - especially those on non-Windows systems, or those without administrator rights. As such, Marble defaults to a per-user location for downloaded tiles. This is also necessary with services which change regularly - such as OpenStreetMap - and need to be redownloaded as a result.

Ah, yes that makes sense. Thanks again.