Visit by the Overheat Gremlin

So.....just as things looked a little safe...the overheat gremlin visited! Well, suppose I expected it at some point, annoying at first (after this afternoon at work was last thing I wanted) but now I've had time to settle its a case of lets fix it.
Description:
When I got the van it had no thermostat (about 2months ago) so had one put in, new coolant etc and all was fine, even good heat from heater. Month later after approx 3 (2x10 mile) trips a week at avg 60mph on motorway it ran fine, cooling as it should an no excess fluids etc.
Over last couple of weeks noticed a lacking of decent heat from heater so bled system and replaced rad caps, seemed better but not top heat so kept an eye on things. All seemed OK...thermo kicked in as normal about 80% then on normal motorway drive needle dropped to 30/40% and stayed there till came off motorway where it rose to about 50% and stayed there. In traffic it went to 80% then dropped back down again as traffic ebbed and flowed etc.
Checked water and noticed other day water res bottle was at full! Didnt hear it splurging water into it and even checked at end of a journey so emptied some out, topped up and rebled system. Was fine yesterday, 1/4" added to.level after motorway drive but didnt seem to get sucked back when cold, fine going to work but comming home temp guage went up and stayed at 80% longer than normal but did come down and stayed down about 50% level after (yes after me sweating and putting heater blowing full blast) . About another 1/4" added to water level. Not had chance to check anything else yet as dark and cold, will have to wait till weekend now.
Next job (which was next to do anyway) is to replace all hose clips I can get to.

Questions:
Should water pushed into expansion tank be sucked back on cooling?
Could changing rad caps have made things worse (proper rad caps)?
Whats my next job after clips?

Really do enjoy driving this van though, best driving position I ever had I think. Didnt realise Astra was sol low to floor and understand why my mother complained each time (however now I have to use a little step for her to get in van...cant win!)

Did you buy a thermosflask from Euro Crap Parts?

Honestly dont know what one it was as I let a garage who has dealt with hijets before (ex Daihatsu dealer/garage) do the first service work/check over as I had zero experience of them. I know they did use Nipon plugs as I saw them on the desk when I called in so presume they used a reliable one. Recent stay at higer than normal temp does suggest thermostat but it was fine previously, but why has it started putting out fluid and not taking it back?

Again, was it a good quality cap? The correct pressure? Check for little leaks, welcome to Warmjet hell ......

Oh dear Raggy...  As Logi says,  the trials and tribulations of Hijet ownership - - at times. If the engine has pushed a quantity of water out into the expansion bottle and you have got an air lock caused by steam then once it cools it can't syphon back in to the system. 
Remember the principles of Hydraulics. It acts as a solid mass displacing a piston back and forth.   It has got to be free from air. What happens when you get air  in the Braking System?  The pedal goes to the floor.  In your case the water goes ON the floor. Hopefully it is a weeping hose which is so often the case. 
When you refilled the system , did you refill with fresh antifreeze?  New anti-freeze will always find its way out of any leaky system. 

 One advantage of red anti-freeze is that you can  see where it coming from. 
Provided that you can get your bonse somewhere near, that is. A mirror on a telescopic arm can be very helpful and a decent LED torch too.   Don't start thinking head gasket yet.  Exhaust all other options first. If you keep it topped up all the time and don't let the temperature get sky high too often you should be OK. It's constantly overheating the engine that can do the damage. 
Hopefully if Sunday is dry for you up there, then you will be able to get down and get dirty. Our forecast here is rain over next 3 days then dry and sunny on Sun-day. Most appropriate. 

Thanks for the explanation, makes sense. Will be getting the clips shortly and hope the weather god is kind to me on Sunday. Maybe getting back to the issue why someone took the thermos out in he first place. As you say, the joys of hijet ownership. Yes I did use fresh anifreeze, so being used more regular now mabe starting to find out its weaker areas.