Replacing the back box

If I can get the bolts out (I don't own a grinder), is this something I could plausibly replace myself with the car on the ground? I don't want to pay £100+ labour to a garage if it's literally just a case of hooking it through a hanger & putting two new bolts through the flange.

IMO 2 options hacksaw or try to tighten them till the bolts snap
On the other hand you could by a grinder for around 20 quid
you'll still be saving money and have one for next job 

Alas I'm in a third storey flat with no garage, so I can't run power for a cheap grinder! I have 18V DeWalt batteries but they want £110 for a bare grinder to plug them in to.

I'm soaking the nuts in WD40 atm (half expecting them to completely dissolve). It looks like this eBay kit for £60 has everything I could possibly need? I'm assuming if I get the bolts out it's simply a case of hooking the new box onto the hanger at each end, fitting the new gasket between the flanges & then just replacing the two bolts? All while trying not to inhale too many rust flakes, ofc.

Get a hacksaw and some spare blades, give the bolts a smack with a hammer, mine snapped off when I did my exhaust. If that don't do it hacksaw them off. It will be done before you know it.
Loads of back boxes on eBay for about £50 tops. You need a 42mm Id ring gasket too or it will leak loads of water from the flange.
A clamp on 12" X 5" 42mm Id straight through muffler will bolt straight on too with the flange cut off. Got mine from 'custom exhausts' on eBay.
Have fun mate

Nuts aside, I found it easier to replace my rear silencer with the rear bumper removed, it gave better access without the need inhale rust flakes.
Not essential and may give more nut trouble if your bumper nuts will not come apart, but mine did easily enough.
Mike

Good tip Scimike, Remove the bumper for access, especially if you got a trailer hitch. Much easier access. Depending on brackets, hitch and back box end pipe design, you might need Some lift like a Jack or ramps or one wheel on the side walk to mangle it in/out.