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How much would you expect it to cost for someone else to come and do it for me?
£1-4k depending on location and quality. To have a raised patio plus for fence work my folks were quoted 6 days labour.
If you do want to DIY it's also worth thinking about getting a labourer to help for some of it. London wage is c.£90-120 p/d. I'm forever recommending this to people* I know who are doing major DIY renovation. You can get x3-4 done with two people.
*mainly wives of husbands wanting to do it all themselves.
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Have you considered SDS, Sustainable Drainage Systems?
This gives you some ideas, and
the legislation covering domestic areas greater than 5sqm.
Looking to lay a patio myself. 15sqm (3.6x4.2).
Reckon I can do it for around £750 in materials including skip bag etc.