Up until two months ago my '96 4.6 was continually suffering from a battery drain which had been going on for the best part of two years. An inconvenience really, the odd booster charge/jumpstart, the same in hot or cold weather - which was always the curious thing.

Recently (funny enough about two months ago) my missus packed in her job with Mercedes Benz and with it went the company C class. Since then I have had no problems whatsoever, even in the recent cold spell my old battery is now fine.

If I think back the problem started around the time the C class arrived. Both cars were parked outside the house, side by side so it appears that the merc was somehow affecting the sleep mode on the rangie. has anyone else come accross this.

Yep, failrly common, known here on several posts whereby peeps have changed their "other" car, even neighbours vehicles, wi-fi or wireless doorbells!!!
You'll find the lektrikery in the merc alarm/fob was partially waking the BECM; as yours would therefore appear to be an earlly unit liable to pick up weak/spurious signals, change the RF receiver to the shielded updated one before you park somewhere and get shafted by a mobile mast or radio-am aerial etc!!