Provided that the system is up to the increased pressure, it is a very simple job. You remove the old tanks and cap off the old feed/vent. You add the pressure vessel, filling loop, pressure relief valve/pipework and pressure gauge. That's it. Normally, the majority (or all) of these components come ready fitted and plumbed in the new boiler. You just stick the pressure relief pipe out of the wall and connect it up, ensuring it terminates at a safe location.
BTW, the 835 is a combi boiler. Is this what you want? Your old system is probably a storage one. If you like baths, you might want to keep the storage system. However, the combi side will be better for showers. OTOH, some high output combis are unable to modulate down particularly low, which counts against them as they will cycle when doing the central heating.
Other posters seem to have covered the points. Professionally you would be looking at about boiler + 1000 (+VAT if the fitter is VAT registered). These boilers are about 1100 quid with the flue +VAT.
So I would expect about just over 2000 all in. BG will be about 3000.
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