It wont be a straight swap, as your current system is probably gravity HW. You will need to change to fully pumped or sealed system. You need to install condensate pipework, which you don't have now. Then you need to install the flue/liner and flue terminal. You'll more than likely need a new cylinder/tank. It will be alot more expensive than a combi boiler, is there any reason that you don't want a combi?
I don't have anywhere to put the boiler ie an outside wall that's free.. I have instead purchased a 3yr old Baxi gas fire and boiler - Baxi Bermuda 50/6.. How easy will this swop be?! I'm hoping it's very similar to the one I have already..