Vokera Linea Max - hot cycling after re pressurising expansion vessel?

I have re pressurised the expansion vessel to 12psi having isolated and drained the boiler. The vessel now holds it's pressure, valve is dry and no signs of water from prv / external drain pipe. No error code on boiler.
Boiler pressure restored to 1bar cold, water clean.
On start up. the boiler will heat correctly to lower temperatures, both HW & CH (60 Deg os so).
However, when the temperature is set to Max, the stystem will heat to around 85 Deg but the boiler then cycles hot - the boiler coming on every 20-30 seconds and running for about the same? Boiler pressure will increase to around 2 bar.
Any ideas much appreciated.