Yes if that's an issued Seiko (generation 2) watch then around £295 ish would be around right - the earlier Generation 1 ones go for £450+. Judging from that MoD sales website they're still being disposed of by the military. As gsimmons says, interest in issued watches has rocketed recently. G10 is most commonly used to refer to the bog-standard CWC or Pulsar watches with (I think) the 541-5317 nato stock number, rather than the higher spec chronos.