non-flowering oleanders

Welcome to GoY, Nakinaki. You don't say where you live in the UK, and the weather makes a difference. Oleanders like heat and sun, really, so if we have a lousy summer, they're less likely to flower.
That said, you don't say how old the plants are now - a certain level of maturity is required before flowering takes place anyway. As you got one flower this year, you'll probably find they flower better next summer - suggest you feed them weekly with something like general purpose Miracle Gro from April through till end of June (not later than that, though).

keep in full sun too..

Thank you Stevebuck and Bamboo.My oleander is 8years old and sited in full sun, but I've only fed it once or twice in any summer: perhaps that's the answer - regular and frequent feeding. Wish me luck for next year !!