These plants should be cut back hard in spring to the wood to keep them bushy anyway - I see you're in Cornwall, so if spring has arrived, and its warm, then fine (or if the plant is inside), otherwise, wait till the weather warms a bit.

Once they get to flowering size, oleanders are very easy and much hardier than they first seem. In Cornwall, if you're near the coast, they can probably stand outside except for the depths of winter. As Bamboo says, they will flower better and look prettier if kept bush like. I know that's true as many of ours are horrible leggy specimens where we have left them to grow too big, while the one that was frosted back to the main trunk flowered beautifullly on new growth last summer!
Don't forget that despite the wonderful scent, the leaves of oleanders are extremely poisonous, and a South African friend once told us they'd been brought up to always wash their hands after handling oleanders in case they put their fingers in their mouths. Probably best to play safe with them.

Good point Bertie, I forgot to mention that - the sap is toxic and irritant, so best wear gloves.

No probably about it really. They are one of THE most toxic plants you are likely to come across.