It was the perfect ride in the Peak District - until some cretin on a motorbike thought it would be hilarious to slap my bottom, both degrading and destabilising me with one gloved whack

Most women cyclists have a grim story to tell: the youths shouting "lucky saddle!" as you pass, the dirty old man at the bus stop asking if you'll give him a ride.

But Saturday was the first time I'd actually been sexually assaulted while out on my bike.

It had been the perfect morning. I'd risen early, determined to hit the Snake Pass, which winds up from Glossop over to Sheffield, before the bank holiday traffic arrived.

There was barely a cloud in the sky as I swooped down by Ladybower reservoir and made my way over to Holmfirth, dodging the Last of the Summer Wine tours as I headed for Holme Moss, one of the most feared ascents in Yorkshire, which will feature on this summer's Tour de France.

It's not so much Holme Moss's gradient but its brutal exposure to the elements which makes it so tough. A few weeks earlier I was supposed to tackle the 1.5-mile climb as part of the Skylark sportive, a fantastic 65-miler which took in a number of the Tour highlights. But by the time I reached Holmfirth, the organisers were urging riders not to attempt to reach the summit: winds were so strong that earlier riders had been blown into the barriers. Some were so traumatised they refused to ride down and someone had to go and pick them up in a car.

All of which made reaching the radio transmitter at the top of the climb on Saturday particularly satisfying. I'd not stopped all the way up and had even managed to overtake some wheezing bloke in the final quarter mile.

So you'll understand that I was feeling pretty pleased with myself when I plunged down the other side and crossed the busy A628 to take the quieter B6015 to Glossop.

As I turned off to the b-road I could hear the roar of motorbikes behind me. A sharp left hand bend was ahead as the road bisected two reservoirs - Woodhead to the left, Torside to the right. I wondered if the bikers would attempt to overtake, but I was pleased when they hung back.

Any positive feelings towards the gang evaporated a few seconds later when they accelerated past, with the first rider giving my arse a good thwack as he overtook. I say he, but I suppose it could have been a woman under the helmet. Could.

At first I didn't know what had happened. I thought they'd tried to push me off, but as my right buttock throbbed I realised with horror that they'd slapped my bum. My hands were shaking as I gripped the handlebars for dear life, trying desperately to stay upright. Once I'd steadied myself I realised I was crying behind my sunglasses. Not only was I frightened but I felt totally humiliated.

Who are these people who think it's fun to degrade a woman as she rides her bike? Do they get a sexual thrill from their leather glove whacking a Lycra-clad backside? Or is it just about asserting power? I bet the bikers had a right old laugh about it when they stopped at the pub for lunch, the odious morons.

When I woke up on Monday morning still fuming I decided I was going to report it to the police, even though I know there is a negligible chance they will catch the pervert responsible. I was too busy trying not to hit the asphalt to get a glimpse of the number plate and I had no information other than the site and time of the assault.

But I thought I might as well - for the record, if nothing else. Derbyshire police were lovely. The woman who answered my 101 call tutted when I told her what happened and after conferring with a colleague confirmed that I had certainly been the victim of an assault, if not also a sexual assault. "Do you think he meant to touch your bottom?" she asked, kindly. I said I thought so: if they just wanted to push me off the road they could have gone for my shoulder instead.

There's a lot of work going on at the moment to encourage more women into cycling: the Breeze bike rides run by British Cycling, the shining example set by Laura Trott and the other pro cyclists on the Wiggle Honda team. I'm part of Team Glow, a women-only club in Manchester whose raison d'être is to persuade women to feel confident on a bike. But how can we get more bums on saddles when we have to share the road with such cretins?