placement of water pump

Hi

Have an old cottage I've gutted and am getting on with the plumbing now. I'm trying to work out where to site the pump in the system to drive the water secondary heating the cylinder and also move the heated water around the rads. My guess is to put it where the water returning from the heat source enters the cylinder. If this is right which side of the pump would I put the off pipe to the rads - before the pump or after? Will I get enough pressure like this or do a need a second pump sited on the rad cycle? I know there was just one pump on the old system, should have drawn a diagram before I ripped it out 

Cheers

Is this an entirely conventional heating system?
Are you changing the system, or just changing the pump, or...

Thanks for reply

The water is heated by a back boiler on a solid fuel stove. It's unpressured, gravity fed but when the water entering the boiler reached a certain temp a sensor on the pipe started the pump. The only part of the system I've left in place is the back boiler with about 3 meters of copper in and out up to where the cylinder was which I’ve also replaced. The old system had the pump where the hot water entered the cylinder coil. It had a T off it to feed the rads hot and the return connected to the return from the cylinder the boiler on the fire.

The pump would have had the effect of speeding up the water from cylinder to fire, but with a T off before or after the pump to feed the rads. So trying to work out if the T for rads should go before or after pump and will a simple T off the main flow be enough to drive water around the rad system – won’t it just primarily drive the boiler/cylinder flow when I want it to drive the rad flow?

Cheers