Thanks all for the replies and advice it was all most helpful. There seemed to be a clear consensus.
Despite the advice I mulled it over, changed my mind several times and decided to stay with C20's with the intention of sourcing an oak pair when a suitable model came up. I then had an opportunity to purchase a pair of ex demo Oak S20's, which along with other circumstances made it right to do and took the plunge. They arrived yesterday!
First impressions are favorable, as others have said more refined and controlled, almost like the musicians aren't trying as hard. This makes them more musical and easier to listen to. The bass is probably the most noticeable change, it is more controlled and they sound much closer to a sealed box design than any other reflex design I have heard with less overhang. The change is more akin to a power supply upgrade on a black box, all the great elements of the C20 are still there, albeit at a higher level. You can clearly tell they are descended from each other.
This isn't to take away from the C20's which are fabulous and I could go back to them in a heartbeat. They represent great value for money and I would suspect given the choice between changing from a NAP200 to NAP250DR (assuming decent source and pre-amp) or changing from C20's to S20's retaining a NAP200 the power amp upgrade would be far more beneficial. In fact I know this to be true having made exactly that change!
I'm probably not fully exploiting the S20's in my system, but clearly the system had more to give as I have seen an improvement.
They have me sorted in the speaker department for a very long time. I'm glad I didn't follow my own advice this time!