Expat needing decent credit score...

[Pitre]

I'm planning to move back to the UK soon and will want a small mortgage (I'll be paying around 80% of the property price).

But I think I'll need a decent credit score to get even a smallish mortgage? I have had a UK bank account forever, always paid my mortgage on time when last in the UK over 10 years ago and have never had any ccj's etc.. I am retired with an occupational pension and soon to be an OAP (within a year).

So what would it be the best way to acquire a decent credit rating within 6 months of returning??