Persuading a tree to change its fruiting habits can be quite tough and may require some persistence. But it can be done.
1. Whatever else you do, make sure the tree is as well fed and watered as possible. From blossom, until it has fruited, it will need to be watered really well in dry spells. A newly planted tree will need a couple of full watering cans of water every two weeks for the first 2-3 years of its life. So the water goes where it is intended, keep a circle at least 1 metre in diameter around the trunk completely clear of grass and weeds. As the tree grows, make the circle bigger... A good mulch of compost/well-rotted manure over the circle in spring when the soil is wet works wonders. If you have not used Rootgrow, feed the tree in spring (before mulching) with a general-purpose granular fertiliser such as Growmore. Follow the instructions.
2. Thin out the fruit on the tree when you have a heavy crop. This is always a good thing to do irrespective of whether the tree is a biennial fruiter or not. You get better quality fruit and by reducing the crop size, you stress the tree less and so encourage it to fruit the next year as well year.
3. The most draconian - and effective - thing to do is to thin the fruit buds in early spring in a heavy fruiting year. This is called "rubbing out" and you literally rub them using your thumb and first finger. You can choose between rubbing out every fruiting bud on every other branch, every fruiting bud on every other spur or between half and two-thirds of the fruiting buds on every spur on the tree. Whichever you decide you will rub out the same number of buds on the tree. When selecting a method remember that you will need to do the same thing to the OTHER half of the buds next year. My favoured method is to do every other branch and tie a bit of raffia or garden twin to the branches I have done to remind me which ones to leave alone next year.
For those who are not sure, a fruit bud is a prominent, usually downy, rather plump bud that is obvious from autumn onwards. Leaf buds, by contrast, are smaller and tend to lie flat against stems.
If you are worried about which buds are which, then as quickly as possible after the flowers open cut every other blossom off.
The purpose of this "halving" is to restrict the heavy crop thereby allowing your tree to have enough in reserve when fruiting is over to produce fruit buds for next spring.