It's not a massive job but you do need to be familiar with plumbing materials and techniques.
One thing to remember, if it's an old galvanised iron tank the pipework will most likely be imperial sized and you will need adaptors for the 3/4" size. (3/4" x 22mm). The mains supply will be 1/2" or 15mm and those sizes are pretty much interchangeable.
I posted a similar question a couple of weeks ago - the bigger problem is how are you going to get your new tank through the lofty hatch ? I gave up on mine as the ones you buy are all too big to go through. the advice in the forum was that I would have to make the hole bigger which would mean cutting roof trusses etc. Not keen on that idea !
My jobbing plumbers had no trouble persuading a slightly flexible black plastic header-tank to fit thro' my narrow loft-hatch. I have difficulty getting my shoulders thro', yet the tank is adequate for supplying the 210 litre indirect cylinder, bidet, upstairs toilet, shower's hot & cold supplies etc. Apparently the key lies in its flexibility.
Plumbers working in my presence have made me aware that metric/imperial adapting at the 22 mm/ 3/4 in, relies on a special olive in the compression- coupling, identified by a green ring of paint in a groove in its rim, so it is generally referred to as a green olive ( I think it adapts the current 22 mm coupling to fit 3/4 in. old pipes.
Round plastic cisterns can be rolled up and can be persuaded to go through a very small hole. You could buy a coffin tank, they come in different sizes so could go through. As for difficulty it depends on your experience, iys seems a bigger job than it is but you need to be aware of imperial sizes and metric sizes, best to buy a couple of conversion fittings and when you take out the old one try these on the pipe to give you a starting point. Another tip DON'T start this job on a saturday or at the weekend as most plumbers merchants close at 12 midday. Good luck!!!