Is £720 normal to replace a coil spring + rear back box on a Smart Forfour (2016)?

Got my 2016 Smart Forfour MOT back with two major defects:

• Nearside front coil spring fractured
• Rear exhaust back box has a major leak (and emissions likely affected)

Halfords Auto Centre quoted me £720 to replace 1 × front coil spring and 1 × rear back box and retest.

Does that seem normal for this car? I’m in Darlington area and think I should maybe take it to an independent garage instead?

From what I can see online, I expected something closer to £250–£500 for parts + labour, so £720 seems pretty steep.

Anyone had/done similar repairs done at Halfords or on a Smart? Worth getting a second quote or is this about right?