Well, last weekend I took my mobile system apart and checked all connections etc. I especially concentrated on improving the ground connection between the Breedlove antenna mount and my car roof.
After re-assembling the system, and parking up on the beach just before dusk, I responded to several low level stations calling, and was very happy that they heard me! I received good reports on my 200 watt mobile signal from stations up to 5,000 km away.
I then changed to my PSK-20 rig, (putting out about 50 watts via my amp), and had a lengthy chat with a station in Tasmania, at 7,000 km distance).
So either conditions were good that day, or my mobile set-up had improved, or both, or luck

I could also hear very faint US stations, in contact with Australian hams. It's likely that if I increase my mast height (when parked up), I can hopefully increase my chances of QSOs with NA.
I consider this very good for my simple Hustler resonator and mast that's only about 48 inches overall length on my car roof.
Now, here's the next plan - 40 meters (static) mobile!
I have a Hustler 40 meter resonator, and I have just purchased a PSK-40 kit off Ebay, which will give me hours of fun and frustration in assembling this kit...
Can those with experience of 40 meters mobile give me some idea of what I could expect in terms of contact distance and time of day? I propose to mount this 40 meter resonator on a 48 inch mast on the front bumper of my vehicle, and then see if I can attach a whip atop of it when I'm parked up.
(Mounting on the front bumper, or generally obstructing the driver's view is quite acceptable here in Thailand - most coach windscreens are almost totally obscured by stickers)
Simon