Finally got to the bottom of this.
I found a Renault Mechanic living near me (just outside Ipswich) who has just started up on his own. He fitted a new crank sensor (you were spot on Uncle Sam), it performs far better now and has been running for about a month trouble free. So much quicker and smoother now, a real treat to drive. If anyone in the Ipswich area needs a Renault Mechanic, I would reccommend this one and I have his number.
And he didn't charge me for diagnostics!