hi mate the fact it was, and is a mass produced small family car ,should go in your favour !!
if it was me and i liked the car i would offer £600 on the basis that you have another 20 or so to look at (nudge nudge)
am sure it may help sway things once the seller sees you have a wad of 600 notes waved under his nose at the time ,as at least he will then know you are a GENUINE BUYER.....
dont forget that its in your favour that there are plenty of fiesta,s within your budget on the market mate ..
hope the viewing goes well for you and post back if you make a deal mate
and as said tinworm is an old fords enemy so check v carefully along the sills and also the arches too