Never say never but the generic weedkiller in generic lawn treatments only affect actively growing green parts of weeds. Dormant bulbs have no green growing parts at this time of year, ergo they should not be harmed.

Would also add that there is little point putting fertiliser on the lawn if it is not actively growing. It is getting a little late in the year for it now.

Thank you both for your advice.
My question related to all year round and not just now.
It's good to have advice through you experts because more than often conflicting advice exists.
One BBC website for instance did advocate fertilisation at this time of the year to "stimulate root growth"
Googling is dangerous however!

If you read the packet on Verdone Weed and Feed I am sure it tells one the optimum time to use the produce (as I am sure it does on all the other makes too).
I don't generally use a fertiliser on our grass, it grows too much as it is. I tend to just use a straight broadleaf weedkiller and that is applied in late Spring and then again in mid summer when the grass and weeds are growing strongly.
Any weedkiller will affect Narcissus when they are in leaf, but not otherwise.

I might be able to clear up the confusion about fertilisation at this time of year re lawns. You can buy what's known as Autumn Lawn products, often combined with mosskiller - these contain a form of nitrogen which breaks down very, very slowly over winter, so that the grass does not get too much, but sufficient to keep it healthy and less prone to disease, and keeps moss away. Well, that's the theory anyway. Autumn products are usually applied September or October, and the next treatment regarding feeding is then around April, when you switch back to a 'normal' lawn food.

That's interesting Bamboo. I didn't know what the difference was.

Forgot about them, again since we do not feed the grass.