To flush you can jut remove the 2 pipes at the bulkhead and attatch a hose to one of the outlets and blast water through each one

to remove
You disconnect the 2 pipes at the bulkhead look behind the engine to see them, remove the metal sandwich plate that the plastic pipes come through through and the foam, the heater box is under the dash held onto the bulkhead with 2 13mm nuts remove these and you can wiggle the box about to get access to remove the heater rad, to remove the rad from the box look at the side where the pipes go into the heater box you will see 2x phillips headed screws remove these and the heater rad will withdraw, it may be tight due to being surrounded by foam, don't forget to replace the strip of foam when you put the new heater rad in as you will get air passing the rad without getting warmed around the edges

I have done them with the dash in, it is a faff but do able, but to make it easier for yourself you may have to dismantle the dash

hope this helps

where abouts in stoke are you? Im an oatcake eater myself, Pm me if you don't want it public