Your local garden centre or supermarket will sell Weed and Feed lawn care products either in granule form or in soluble or liquid form that you water on with a watering can.

Its a matter of personal preference but I find that the granular form works better for me.

It takes a while for it to work but that is probably a reasonable first step. It sounds like it will need a few applications over the summer if the clover is very dense.

we also use 'feed and weed' granules and with good results. over 2 months or so the grass really improves and less weeds.

This is intriguing - Inverglen and Seaburngirl, are you saying that the standard Weed and Feed granular lawn products are effective against clover? Because this has not been my experience. It does check it, but doesn't kill it. Unfortunately, the only product that definitely did was withdrawn from the market 15 years ago.
I resorted to digging out the clover in September and reseeded the areas.

I don't think there is anything on the market for the amateur grower any more.

Before you could buy selective 'broad leaf' weedkillers disigned for such problems but not any more I'm afraid.

Farmers have one which has been in the news of late made by Dow chemicals and it contains aminopyralid which does what it says on the tin BUT!!!!

It is designed to kill broad leafed weeds in hay and any residue that remains will not affect grazing animals but it can affect gardeners as I and other have found to our dismay (to put it mildly)

What happens is; the weedkiller goes through the animals digestive system and is urinated/excreted into the animals bedding.

This bedding is sold on to the unsuspecting gardener as "Farmyard Manure" but what the gardener does know until it is too late is; the manure contains " weedkiller" which at best affects cropping, and at worst kills the plants.

Sorry for slightly going off the subject but I thought I would just mention it!

There is more reading & pictures here;

http://www.thegardenersalmanac.co.uk/Data/Aminopyralid/Aminopyralid.htm

I find doing 3 things helps to reduce then eliminate clover in lawns:

1.High nitrogen feed in spring
2.Rake the areas where the Clover is before mowing. That lifts it for the mower to get to it.
3.Cutting the grass regularly and lower where the clover is - it tires it out if you remove all the leaves.

Picking it out by hand also helps, if you get a patch early enough it comes out easily and in one go.

Using cardboard to darken the area for months does not work - just sends it inot hibernation mode.

For a lot of people it is the greenest part of their lawn and they live with it...

Look for Yates Bindii & Clover Weeder Concentrate.

How It Works

MCPA and Dicamba are absorbed by the leaf, down through all roots of the weed. The two weedkillers allow a broad spectrum of weeds to be controlled. This product works similarly to other hormone weedkillers, i.e. the weed is forced into an excessive and disordered growth pattern which results in weeds curling, yellowing and dying within 4 weeks.

Ingredients

Dicamba, MCPA

Directions for use

Mix 30ml in 10 litres of water and apply over 20 square metres.

Apply when weeds are actively growing and rain is not expected. Use a watering can with sprinkle bar attachment and/or a coarse garden sprayer. Do not mow for 7 days before or after application.

Effectiveness

Poor results could be due to: i) Applying when weeds are not growing actively; ii) Watering or rainfall within 12 hours after application.

Precautions

Do not apply to lawn if soil is dry, conditions are very hot (30 degrees) or if lawn is suffering any sort of stress. Eg fertiliser burn.
Do not apply if rain is expected within 24hrs.
Do not spray on windy days, avoid spray drift onto wanted plants.
Do not apply to foliage or near roots of trees, shrubs or plants.
Do not apply to new lawns less than 3 months old or clover lawns.
Keep pets away until lawn has dried.
DO NOT USE ON BUFFALO.
DO NOT USE MORE THAN RECOMMENDED RATE OR LONG TERM DAMAGE COULDRESULT.

Bamboo says that weed and feed products do not kill the clover. OK. My experience is that it has a clear effect of knocking it on the head so that it does not take over . But kill it off completely , maybe not. But its about the only product that we can try .

thank you all very much,interesting

Inverglen, I didn't say it didn't, I said that's been my experience - I was hoping you'd tell me it definitely did kill it. Sadly not, apparently.
The Yates product mentioned by Tmh above appears to be unavailable in the UK.

Verdone extra will kill white clover and lesser trefoil

Weed and Feed doesn't appear to have any effect at all on our clover unfortunately.

Thank you Anchorman. On to my shopping list goes Verdone extra.

I'm not overly impressed by most granular weed and feeds. They kill dandilions and certain other weeds well but don't touch lesser trefoil which was rampant in many gardens where I work.

I use a granular weed and feed to kill the basic weeds and feed the grass and about 4 weeks later apply Verdone extra which kills the weeds the granular feed doesn't touch