How can I make the heat generated from a multi fuel burning stove not go up the chimney?

It's hard to picture the exact situation but I'm uming the pipe comes out the back of your stove and then goes vertically up you chimney flue. As the pipe enters the flue behind where the mantle piece should be you should have a closure plate. It is sometimes called a register plate. It should fit exactly into the flue and have a 6 & 1/8" hole cut into it for the flue pipe to p through. The small gap round the pipe should then be packed with a length of fire rope to give a good seal. The register plate should be made of steel but if you aren't in a position to cut and drill steel there is another product which is called fire proof board or Asbestolux. Don't worry it doesn't have asbestos in it! Most builders merchants stock it. It can be cut with a jig-saw and drilled fairly easily.It is fixed into position with screws and rawl-plugs and then the edges sealed with fire cement. To be honest it's really a job for a HETAS registered solid fuel installer because of the risk of carbon monoxide poisoning. Once fitted the register plate will give your stove the best heat generation available. If you are at all unsure please get professional advice as your health and that of your family may be at risk. And don't pack anything around the pipe or you could risk a fire.