any ideas on this ??

Have you checked the PAS reservoir in the engine bay? Make sure there is sufficient in there for a start... 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

When I start the car on the drive ,the steering seems heavy,then when I rev the engine to move ,the steering gets lighter with a small shudder. I wonder is it something to do with oil pressure/flow to the rack or from the power steering pump?

Rover T - I'd suspect that the pump would be to blame for either of your suggestions.

Zippy - A number of things could be causing this - loose drive belt/faulty tensioner, faulty pump, sticky track tie rod ends, faulty steering rack or possibly the universal joint. I know a few people have had trouble with the UJ. Thinking outside the box - have you checked your tyre pressures lately?

Does the steering get lighter when you drive it or rev the engine? Do you hear any sounds from the engine bay when you turn the wheel?

pump is going out, same issue happened to me

rebuild kit or a used pump run about the same cash, i went with a used pump from the salvage yard, selected it up for about 30 dollars, with less than 50k on it