As TW effectively indicates, Premium Bonds only remain in the draw for 12 months after the death of the holder (with any winnings during that period being paid to their estate). Thereafter they can be cashed in by the executors of the estate (or by the beneficiaries that they've been passed to) but they can't win any prizes.
I'm in a similar position, in that I've got a few Premium Bonds in the name of my late mother, who died in 1985. They're effectively worthless, as they can't have won any prizes since 1986 and it's not worth the hassle of cashing them in just for a pound or two.