Sorry to hear about your problem. I'm not an auto paint sprayer but when I worked for Eurofleet collecting hire cars at the end of lease they sent us on courses to train to assess damage and costing of repairs/replacement, and if I remember correctly metallic silver cannot be touched up to make invisible repair, there is allways a darkening at the edges. You may have to go with the excess as prep and full panel resprays are going to cost a lot more.
I agree with the darkened edges - never going to be invisible.
But, probably no harm in trying a small section. Dab thin layers of paint into the centre of the scratch, I use a cable tie for this (the ridges retain the paint and it has a nice point). Then, when dry after a couple of days, wet-sand with 1200 grit wrapped around a wooden stick. Be very careful not to sand through the surrounding clearcoat. Polish with a non-silicone cutting compound such as 3M Finesse-It. I don't usually attempt to apply a clearcoat, as the basecoat polishes up quite well.
I expect this will cost at least 20 pounds in materials, so it could be worth saving towards the SMART repair instead. However, if you're unemployed as I am at this time of the year, it's definitely worth a thought (I cut back on chocolate and avoided buying alcohol for me or anyone else for three months, so I could replace my suspension arms and bushes
I have some scratches on my metallic silver 156 and need to get around to trying this on the silver, I'll report back. Already I happen to know that there are two very-similar metallic silvers - the earlier is a fine texture and slightly more 'blue', the later is a coarser texture and slightly more 'yellow'.
I have touched-in scratches on dark colours with fair success in the past - considering that on a dark colour, a white scratch is much more noticable, and a metallic-texture mismatch is less noticeable than for silver.