Lawn was set 28 years ago. The lawn was planted in June and a prolonged dry period followed. Grass was slow to appear,but weeds took off immediately. I suspect that sub soil was used, rather than top soil, when the lawn was set. In Winter or during damp summers, moss appears in most parts of the lawn. Over the years the lawn hasn't really improved. Grass seems to suffer greatly even after a short dry period. Should I now take up the lawn completely and bring in new top soil, should I dig it up, clean off weeds, moss etc and lay new seed again or this there a Plan C? What is the best time of year to attempt this work?
Do not worry about the use of subsoil versus topsoil. I've seen sod laid out over 5/8 minus gravel in our yard and THRIVE. Amazing.
Are you set on seeding your lawn? I would call around to get bids on a lawn seed spray company. Gosh I am hoping you are in the northern latitudes with cool season grasses. Those I know very well. I prefer using sod, worth every penny and I've never seen that sod was a worrisome expenditure or even a noticeable expenditure.
If you have warm season grasses, use St. Augustine. I've got the guy who knows St. Augustine very well...his videos that I have perused and was very impressed by.
Just wacking down the grasses and weeds (before producing seed hopefully) then raking and rolling !!!!! filling in depressions then raking and rolling again is all you need to prepare your lawn for sod...again.
If you send pictures we could be help you decide to use a sod cutter to remove the top 2" of weed seed filled soil and organic debris if necessary before new sod. Sod is great as it smothers weed seeds quite well.
If you are able to get a vigorously growing grass crop that crop will be able to out compete any weeds. If you are using a cool season grass seed mix, or St. Augustine, you have to learn maintenance techniques such as mowing HEIGHT. How to water correctly to train the roots to be drought tolerant. How to fertilize correctly. How to sharpen your blades sharp sharp sharp...it isn't tough to have a magnificent lawn, just need to know how this monster the lawn works!