Have you tried the ukcampsite for sale section, or try Pre-loved? If you wait a couple of days there will probably be a small flurry of tents for sale after a crappy wet weekend camp and subsequent row!!
We have a vango tigris 800 surplus to requirements. I wouldn't recommend it as a tent though, not enough windows or light, and a bit flimsy.
That's why it is surplus to requirements
Give it a couple of months and there will be loads for sale 2nd hand. Get an idea of what layout you want and hold tight. The first tent we bought, the vango, has sleeping areas at either end which didn't work for us at all. If we had done our research properly we would have got a tent that suited us better from the start and saved some money.
I want one with sleeping either end though as the dc's drive me mad and chat all bloody night Really wanted something for the summer holidays.... Dh wants to get the crappy halfords one so thought If i got one first he couldn't
Hydrostatic head of 1500 is adequate. We only camp in the summer anyway, and as i said earlier, our tent has never let rain in. MOD consider 800mm as waterproof, so 1500 should be fine.
As for the ground sheet, I prefer them loose. When packing the tent away its so much easier. With sewn in groundsheet, you have to get all the bits of grass etc out and make sure the bottom is dry before packing.
Its all down to individual preference, personally I like to make life as simple as poss when camping.