I can't not post this (but I suppose you've all seen it!):
I used to change wheels as shown on my CX, BXs and Xantia and, I'm pretty sure, on my facelift C5. (Can't understand why .........!)
Or, to put it another way, your tank holds just over 13 gals and, at 50mpg, a tank full should do about 650 miles, with the warning coming up when you have about 1.5 gals left.
I don't know whether you can do so on the C4 but on my C5 I can set up a display to show miles covered, MPG and range (in miles) left in the tank. If you fill the tank then Zero the readings you should be able to see the MPG, how far you've driven and the estimated range when the Low Fuel warning comes up.
"tyres are lasting around 9k on the front which is not bad for a heavy car with pwr steering". Doesn't seem a lot to me. I get around 20K from the fronts and 25K from the rears on my 1.6HDi (always Michelins) and, if I'm driving a lot in France, about another 1.5K that I put down to the much smoother roads there. (Used to get over 40K with Michelin Xs on my Triumph Herald!)
If the clock is from the starter solenoid then there's some voltage in the battery but maybe not enough as Paul says.
I'd want to know why the guy you bought it from disconnected the dash to hide the abs, break fluid and engine lights. I assume it's been reconnected so there may be a fault there if the battery keeps discharging.
Check the voltage whilst it's connected and see whether it goes higher with one of the terminals disconnected. (I've had a battery that read 12.3 volts off-load but dropped to just over 10 (one dead or high resistance cell!) when on load (engine off but headlights on).
Also, if it's always (as soon as you stop the engine) in Ecomode, the alternator may be worn or wearing out and not fully charging.
Best of luck!
It sounds like a head gasket problem to me. The Haynes manual (4745) gives instructions how to change it and this should be a cheaper and easier option than trying to shoehorn in another engine. Get the manual and have a read. (The manual doesn't include the 2.2l version of the engine but it only differs in having a 8mm longer stroke so...)
Hope this will help you as it's probably the same for Peugeots and Citroens and I think the Digibox is like a Lexia (?) http://www.peugeotforums.com/forums/electrical-54/how-reset-dpf-fluid-additive-level-counters-pp2000-21676/
"Now battery manufacturers say that a sealed battery isn't need any maintenance ,but that isn't true is battery manufactures as the electrolyte evaporates and part s of the plates become uncovered and then a battery doesn't perform very well .if you are able get the plugs out you can top up the electrolyte and fill up with distilled water and a good charge and it will last a little longer ." I check mine and top up with melted ice from the freezer about every couple of months. Have to be a bit brutal with a wide tipped screwdriver to unscrew the (filler) plugs! My first ( from new) battery lasted about 9 years before a cell went. Charging the battery produces hydrogen and oxygen frmo the water in the electrolyte, thus the level gets lower.