New Boiler or repair which will be my best route?

I have a worcester greenstar combi boiler, it was fitteed when I purchased the house 18 months ago, although on central heating it seems fine and also domestic hot water in the kitchen, the problem is the hot water upstairs, if the radiators are not switched off then the hot water will bleed into the radiators heating them as you run a bath, the water then available for a bath is lukewarm and scarce in quantity.
I recently had the boilers serviced (probably it's first time destroying the worktop that had been fitted above the boiler and it's top access) the engineer quoted around £300-400 for a new exchanger unit or 3000 for a new boiler, I considered both quotes a little on the excessive side, so I was left with a £70 service fee and a stench of kerosene in the house that has lasted for months. I'm tempetd to purchase a Grant condensing boiler at around £1800 inc vat. but what sort of cost will it be to install., and am I proceeding down the right road or should I opt for the repair of the old boiler (about 13 yrs)