i now need to decide which one to go for.
basic condenser or combi,
which is the most cost effective to run,
Also if combi is fitted does gas man remove old hot and cold storage tanks and circulating pump and diverter valve or does this lot stay in place
It depends on the size of your house and family. If you're wanting to run 2 or 3 baths per day, or more than one mixer shower at a time - then a combi wouldn't be much good.
If you have an average 3 bed house, with one bathroom or only one shower or bath would be used at any one time - then a combi would be ideal. This would be more efficient as you're only going to heat the water you use - not a cylinder full that may not be used.