For the past five years, Henry Furniss has watched as his father's vascular dementia has gradually robbed him of his memory, speech and mobility.

Retired London banker Valentine Furniss, 87, no longer understands that Henry is his son, or even a close family member. He struggles to formulate sentences, and needs full–time care at home to perform basic tasks.

Tragically, it is a situation which has become wearyingly familiar for Henry's family.

Valentine's father John, Henry's grandfather, died of dementia in his 80s. And John's younger brother, who had been a Battle of Britain pilot, also developed the condition, which affects nearly one million people across the UK and is the nation's biggest killer.

Henry, 51, could have been forgiven for assuming that dementia was just something that ran in his family and that he too would inevitably face the disease himself.

But the former personal trainer, who lives near Guildford in Surrey with his wife and two teenage daughters, is taking a different view. 

Buoyed by the growing swathe of research that suggests nearly half of all cases of dementia can be prevented, Henry is determined to change the pattern that has struck down too many men in his family.

Retired London banker Valentine Furniss, 87, no longer understands that Henry is his son, or even a close family member

Henry is determined to change the pattern that has struck down too many men in his family

'Everyone thinks their risk of dementia is written on the cards from the day they're born, that it's inevitable you'll get it if it's in your family,' Henry says.

'We're also hardwired into thinking it might never happen to us.

'But if you wait until then, it's too late. We now know there's a time in your life, in middle–age, when you're defining your final chapters – you're sealing your fate.

'For my dad that was in the 1960s and 70s, when we didn't have any concept of healthy lifestyles and how they could prevent disease.'

Everyone experiences dementia differently. Use this checklist to help you make a note of your symptoms before you talk to your GP.

Henry adds: 'I have a different choice to make, and I want to make it not just selfishly, to prevent dementia, but to protect my family against the emotional distress this disease causes, and also the huge financial strain of paying for care.

'If you know you could do that, and live many more years of healthy life without the burden of disease, why wouldn't you?'

Dementia, which claims 76,000 lives a year in the UK, has long been one of our most feared diseases.