Shredder - we had exactly the same situation a couple of months ago.
I would reccomend getting a new self condensing combi unit. This means your storage tank and header tank will be taken out - no need for them.
For us we got everything done including cleaning/flushing the system, new controller, new incomer, new flue, building work and replacing some pipework which was blocked up etc. All in it cost £1400 including VAT. Our plumber was a machine and did it all in a day. Instant hot water is great. The new system is far far more efficient and uses way less gas. It heats everything up way better and the thermostat is a lot lower than it used to be.
Hope that helps.