Peter, you have two options.
1) Fit a modern alternator that is regulated to a maximum higher charge voltage typically around 14.5 volts.
2) Fit an external smart regulator/booster to your existing alternator.
Sterling Power products offer a range of these which electrically piggy back the field circuit of your existing alternator. They are configurable to suit a range of battery charge regimes as well as providing multi-stage charging, including float mode which can increase battery longevity.
TBH I'm surprised you have found or are convinced your present one is regulated to only 13.8 volts at normal engine cruising speeds, given it's modest age. My near 60 year old LR with dynamo charging gives over 14 volts for example.
Finally remember it's the battery to a large degree that limits the alternator output voltage. It's only when close to being fully charged that it allows alternator to hit it's regulated limit as current draw tails off. An aged or faulty battery may be drawing so much current (whilst getting quite warm) that this limit is never hit.
