Halstead Ace HE24 overheating FAULT CODE 1 - Help Please

I'm relocating my kitchen s/cock and, as I've done before, went to close the 76 yr old pavement supply cock. After a few turns it stopped turning but water was still flowing, next I unwound it to consider plan B, however the spindle had seized and the entire mains cock came off leaving an 18" high water feature on the pavement and a trickle in the kitchen so I switched the boiler off. Severn Trent arrived and plugged the leak with a long plastic rod but my trickle stopped and back flowed (could feel a vacuum on the garden tap which I'd left on to spit out any stones or debris). They removed the plug and eventually replaced the valve using a 6 foot socket spanner. Prior to this the boiler was working fine, but when I fired up after the pavement leak was fixed the boiler came up with error code 1 (water temp >105c).. suggested action (1) Check no air in HE / CH system (2) Check plate HE not blocked (3) Check diverter valve operation (4) Check Flow / Return Thermister.

I've (1) Bled the rads & boiler top vent (2) Removed small stones from cold water supply conical filter and checked the Plate HE filter (3) Removed the water pump to view the diverter valve (which rocks back & forth) (4) Checked the operation of the diverter valve motor (out of housing) (5) Checked the thermistors ALL of which are about 5.5k ohms at 20°c.

On power up the fan modulates but the CH flow doesn't do it's usual purge.

I can get hot water to fill a bath or sink but within seconds switching off the tap the boiler shuts down with error code 1. The central heating does not work at all. Going to try to remove the crud coated auto vent on top of the diverter valve in case that's causing an air lock.

When the boiler goes through the start up procedure the CH pressure gauge goes up and down by about 1/4 bar

Turned out to be a stuck flow sensor (in the FLOW mode)