I have got various synth and merino base layers which are great in the autumn/winter but not when it is warmer. Anybody recommend a base layer that is nice and light and wicks away as efficiently as possible?
What’s the newRab MeCo fabric like? Or am I better off sticking with synth and putting up with the stink?

Take a look at the base layers for road riding. A lot of the summer ones are really light and are not much more than a comfort layer under bib shorts, absolutely full of holes.

The Craft Cool Mesh Superlight baselayers are excellent – come in a ‘T’ or vest.

Can’t beat Craft for baselayers

+1 Craft.

Arc’teryx Phase in the lightest weight they do is very, very good. The summer weight Berghaus stuff is okay too. I really like Rab’s MeCo as a sort of halfway house between merino wool and a synthetic. Has the best qualities of both – ie: wicks and dries fast, feels comfortable against the skin – but doesn’t wick quite as well as the best synthetics.

Synthetic stuff treated with Argentium or Polygene or similar silver-based permanent treatments is pretty odour resistant by the way, though it varies a little depending on the user.