Oh dear. Well, it rather depends on what quality of lawn you want at the end. I've done lawns many a time for clients, I hate it, but I've done it, and what you should do, technically, is prepare the area thoroughly and properly for best results.
That means digging out any weeds, particularly perennial ones with large roots, digging over the whole area, removing bricks and any large or small debris you may find. Then you level it (I used to use an old floorboard with string, and drag it across the area, but any flat, long object will do, I once used an old stepladder). Then you walk all over it, closely, on your heels, when you will inevitably reveal any 'soft' spots which are lower than all the rest. Rake up and re level, then walk over it again, rake up, relevel. Let it settle for 4 or 5 days if you have time, then broadcast a granular, balanced feed such as Growmore all over it, till it looks like fallen blossom. Then lay the turf.
I bet that's put you off.... Many people, though, do just what your turf supplier suggests. If you are happy with the even-ness, degree of weed invasion and level of the current lawn, you could do what he says, but of course, the new lawn will sit a good 2 inches higher than the old one. For interest's sake, Roundup will not kill off clover and certain other lawn weeds in one application, and it has an impact on the environment. It was once claimed to be 'environmentally friendly' and 'bio degradable', tags which they have been obliged to remove because it certainly isn't. If you choose to use a weedkiller, use ordinary glyphosate - same killing ingredient, but without the added surfactants. Its the combination of ingredients in Round up which make it environmentally unfriendly.

thanks bamboo ,i didnt think it would be as easy as he says ,acctually highering the lawn would benefit my purposes ,i have a gallon of clinic ace weedkiller here but i am reluctant to use that as it is very strong i know i can dilute it more but if i leave the old grass on i wonder if it would affect the new turf roots ? thanks again cheers pete

I see its industrial strength glyphosate, but I couldn't find info on other ingredients - if its just glyphosate, that works 'through the green' so to speak, meaning you apply to plants growing strongly and it penetrates through the leaves and into the stems and roots. In theory, there should be no residue in the ground, but you should wait at least a fortnight before laying turf over the top.

excellent again bamboo many thanks ,cheers pete