Well, you were quite right.
So, from no problems for a year or so, I started having chain slippage (on both bikes) - and this is down to the lube 1000%.
The slip on the tt bike was so bad the other day, that it must have moved the derailleur cage (which is not too stiff as it uses an after market spacer to work with 56T) and on the up-shift it was flexing inwards even more.