Help.

Any advice as to where to look for advice regarding building a small 00-gauge layout for my grandson [9yrs old].Do not wish to spend too much as he may well loose interest sometime in the future, he's very keen at present.Thanks

A Hornby train set would be the place to go. Either new or second hand as you can pick up a larger or smaller set depending on budget. Will give you a controller and some track to get you off the ground. 
Model shops can give you some great advice if you have a local one? If not I highly recommend Great Eastern Models, they deliver and if you call them they will talk you through any aspects. https://www.greateasternmodels.co.uk/

Hi Roy

Agree with whats been said so far.  I would go for a freight set - trucks are great for filling with sweets, soldiers, lego etc whilst coaches don't offer this.  A few extra trucks (second hand) can add to the fun, especially if you have a siding or two.

If you can afford them, swap out the Hornby points and get Peco Set-Track replacements.  Same geometry but better made and better for short wheel-base locos.

If you can, a crane wagon adds more play value

Barry