I don't know what I did to appease the transport Gods, but it worked.
I'd allowed two hours to get to Bluewater from my sister's (90 minutes in case of heavy traffic plus 30 minutes 'just in case') but it was done in under 75 minutes and that included knocking back the cruise control by 5mph once past Chelmsford. A little wander was had (I stayed away from the
Ribble Cycles showroom

) and a cup of coffee while waiting for the others to arrive and this was followed by a very enjoyable near 3 hour lunch and natter.
With late afternoon traffic the return drive took slightly longer, but nowhere near as long as expected. If only it was always like that.
I'm still stuffed from lunch and haven't even touched the Sicilian Lemon Cheesecake Slices spotted on yellow sticker while walking thrugh M&S on the way out.