It depends what you want, nice seating area /patio/ lawn/ raised beds/ grow veg.
Sounds like a lot of hard work.in the main body of the garden level off pack on some new top soil and lay to lawn nice. Keep the weeds at bay with
the seasonal weed & feed.
sort out some bed areas and plant to suit your soil and taste.
suggestion:- tall plants or climbers near the back, shrub where you want them and flowers at the front.
tip1) do not be too regimented as plants do not gown in straight lines.
2) remember the more plants you put in the less room for weeds to grow.
3) look for good spreading plants.
4) between your plants -get all the bulb in for a great display in the spring.
5) if you make a patio remember the big pots in the end of season sales will be beautiful for the summer plants next year.
post some before and after piccy's its always interesting to see what people do with the space especially from scratch.

I can't see the point of a membrane under a lawn - because grass is mown regularly, the majority of weeds give up. Exceptions would be docks, bindweed, and the flat rosette type weeds like dandelions. I'm also wondering what you mean by 'hard' grass...

70 sq metres of just lawn is a lot and won't be very interesting, but however much lawn you want, if there really is no salvageable turf left, then scrape off what's there, dig it over, removing weed roots as you go, rake over and level and either lay turf or seed. Turf can be done in October if the weather is reasonable and the ground not too waterlogged.

Membrane is a good way of spending money, but digging and turfing is better and cheaper. Weeds will appear in a lawn regardless of whether there's a membrane or not as the seeds ow in from elsewhere.You might as well improve the soil you have as import someone else's. Dig some of it and see what quality it is. If you feel it needs topsoil it would be better added to what you have already than put down on a membrane. If you try to cut corners at the beginning the results may annoy you for years afterwards.

To Johnrobertso, bambu and steragram,
Thank you very much for your advice. I thought so all along, but I needed to make sure the membrane was a preposterous idea. I should say this was a contractor gardener's proposal. It would be very difficult for me to dig the existing grass and I am thinking to hire a tiller - I am sure I would not be able to handle a heavy rotavator - Have you ever used one of this electrical tillers to dig compacted soil?
Thank you again.

I haven't, no - I'm not a great lover of this kind of thing if there are weeds present - things like ground elder, bindweed, etc., will regrow from the tiniest fragment of root, and those electrical tillers/rotavators do a very good job of chopping up all the roots and spreading them far and wide, thus increasing the weed problem.

Suppose you treat the whole area with roundup, and then when it is bare have a contractor till and level it for you? Then you could leave it to settle over the winter, then take out any new weeds and turf it next April?

Thank you Bamboo and Steragram.
This is what I am doing now: I am treating the borders - I used roundup, glysophate and weedol in three different applications, although I cannot see much results yet. If I find it, I may use paraquat also. I intend to dig the borders as much as I can with some help first, add enough topsoil to rise the beds, mix some sand - I do not know the proportion - and put some climbers and evergreens choosing a mix of foliage and colours, so to define the surrounding areas. I may follow your advise and treat the lawn area as well: I do not know about asking a contractor again:I consulted three. Here in North London those available and somehow affordable, do not seem to be up this kind of job.

I'd add one thing - don't bother with the sand in your borders. Topsoil's okay, but remember also to add plenty of well rotted manure or garden compost or leaf mould, anything you can get hold of like that. That'll do the soil much more good than sand, and your money would be much better spent on that.
And the trouble with cheaper gardeners is often that they're not qualified, just doing gardening to earn some money because its seen as unskilled labour - they usually rely on the owner to tell them to cut this, dig that, etc. They can be useful for labouring purposes if you know yourself how something should be done. If they're part of a team, there's usually someone at the top who does know what to do, but they're more expensive.

I'm pretty sure Roundup is just a trade name for glyphosate, but you can check that from the label. There is no need to use all three, just be patient. Glyphosate works by absorbing the chemical into the leaves and then transporting it to the roots. You should use just one, in the recommended dilution. You won't see the results for several weeks. Its not a good idea to overdose on any chemical in the garden. More is not better.

Weedol is a different formula and I don't know anything about it except that it is quicker.