I would definitely take a hard look at the economics of running two cars. Hum drum economy aside, there's no reason to keep the miles off the megane since the residual values of Renaults are quite low anyway.

A diesel Berlingo could be the answer as Bromptonaut suggests. If you would like a 4 x 4 at weekends, would a Skoda Octavia 4 x 4 fill the bill? It ratther depends why you want a 4 x 4. An Octavia with the 1.9 PD or 2.0 CR diesel engine (avoid the 2.0 PD) could do as your only car.

I had experienced a similar type consideration. I changed jobs last year and with it took onboard a 300 mile weekly commute. So should I buy a cheap to run car for the communting whilst keeping my 32 (ish) mpg 2 litre petrol Primera for weekend and famliy use only?

I came to the conclusion the cost of running a second vehicle - insurance, maintenance, tax and MOT and depreciation would far outweigh the savings a 50 mpg car would achieve. My last Primera kept going to beyond 170+k miles at which time I sold it, and I will keep the present car to (probably) a similar mileage - with deprciaton being a hidden cost factor. Of course a spike in fuel costs might change the dynamnics of the costs.

Also consider the driving experience, it's a lot of time behind the wheel, for instance I find cruise control great for reducing the stress of fast A road driving.