I have got a heater that I will plumb in after SVA. I am going to the Donnington show where I'll pick up a few things that I need.

I was going to plumb the heater in "in parallel" with the radiator but I have just thought that if the heater fan is off, the heater will just be a short circuit across the radiator for the hot water. What do I need to plumb into the heater circuit to prevent this and can you get a dash mounted control for it?

Modern engines usually have heater take-offs built into the block, leaving the radiator run as simple as possible. Having the heater core in parallel might confuse the thermostat.

I am using a bike engine. There is no heater on the donor.
I'm hoping to get a flow regulator witha dial or slider for the dashboard. That way I can turn the water off when the weather is nice and only turn it on when it's cold. I could just fit an inline tap but I'd have to stop and lift the bonnet to adjust the heating.

Honda civic / CRX have little valves on one side of the heater matrix and use a stiff wire rod to open and close the valve fron inside the car. maybe you could try one of these?

RS Components do a 12v solenoid actuated water valve which would be ideal in this situation - just have a switch on the dash.