hi i have a 20yrs -25yrs baxi burmuda boiler when it first comes on in the morning just have hot water on not heating on now you get a lound knocking noise which lasts for about 3 mins

It can be converted to do this

The noise is probably Air in the System or the system has a blockage(s) or Heat Exchanger is scaled up restricting flow. I would bleed all your radiators to make sure full of water, open any air valves and make sure no air in system also. Otherwise call in a Plumber to check the system. Reason why you cannot have Heating on its own is because it has to have Hot WAter on to switch Boiler on. You can have a new programmer fitted and rewired so that you could have either working independantly, this would be completed by an electrician. Bill Pipe Dreams Plumbing Services

sounds like you have gravity hot water and pumped central heating possible pump failure plus boiler probably scaled up time to think about a new boiler