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Having looked at your photo a few times now, I'm in agreement, with the proviso that the caterpillars of Square-spot Rustic, Xestia xanthographa, and Six-striped Rustic, X. sexstrigata are apparantly indistinguishable, according to Chris Johnson on a different site. Your larva does have a particularly bright and clear cream lateral line which isn't spot on, but this may be accounted for by variation or if the larva has only recently been through a moult maybe.
I thought it looked similar to one I saw in February last year, which was identified as Xestia xanthographa:
Lat/Lng: 51.0, -0.4
OS grid ref: TQ1431