Hi Hank, wouln't do any harm for the drainage, but would do nothing to stop them coming back, Derek.

Oh dammit. Thanks D, I thought it was too good to be true.

Hi Hank. Re dandelions
I had the same problem when I moved a few years ago.
My answer was to get a short length of drainpipe, about 6" [150mm] long and to fold up the dandelion leaves and to slip the pipe over them and then to spray into the pipe with a total weed killer. That way you only spray the affected area. It worked. It did leave many neat round dead patches on the lawn but through one season I cleared the whole lawn. Depending on the weed killer you can reseed in about 2 weeks.
Now, I use another old trick for any which slip through my defences. I pour a small mound of common table salt into the dead centre of any dandelion I come across. That will work and kill them of pretty quickly. Also, if you have the time and patience you could always pick of every leaf. That way they cannot photosynthesise and will eventually die, but you may also lose the will to live with all the bending and plucking. This time of year I diligently pick and bin every yellow dandelion flower I come across. That way you will prevent seeds from blowing about.
Good Luck
SouthportSmith

We do the "dandelion job" every day at this time of year too. Salt works well, but in lawns doesn't it kill the surrounding grass? I'm hoping Weed and Feed will deal with most of them - the area I treated last year isn't as bad as the rest but it works out expensive over a big area..

Thanks guys, I'm now pretty sure I can get rid of them now.