I am finding that if I have been freewheeling then start pedalling again I sometimes get a loud clunking sound.
My chain and cassette have been replaced quite recently and I was getting this sound prior to replacement.
Any ideas or advice the only thing I can think of is the rear derailleur.

Sound like your freehub need it grease soften up ( the grease is get old and dry ) or there some dirt in it . The fix is easy , remove the cassette , spray some Wd-40 into the hub as you spin it , let it dry then add some light oil to the freehub like some tri-flo as once again you spin it . If this doesn't solve it then you need to replace the freehub .

Sounds normal.

Sounds like a possible ratchet pawl problem where one pawl isn't fully settling into the ratchet ring's teeth until torque is applied.

What kind of freewheel ratchet or cassette body are we talking about here?

It could be in need of lube or it could be a bad pawl spring, but don't spray anything into the hub where you might wash fine grit into clean ball bearings. You need to know the mechanism you're dealing with!

Thanks for the suggestion hadn't thought of that. It is a Shimano cassette body attached to Dura Ace hub with a Tiagra cassette.

I don't think we have enough info to diagnose, let alone guess what the problem might be. When you say it clunks, do you mean something that occurs separate from the normal sound when you start to pedal and the pawls engage? If you are in a high gear and start pedaling hard after coasting you will normally get a clunk sound/feel when the pawls engage. We need a better description of what happens, what gear(s) and how long it has been occurring. It's always helpful if possible to switch out a rear wheel temporarily to isolate the problem, but of course be careful to readjust the derailleur.