If i take a BIG cutting from a big bush...

[spikemomma]

And shove it into water, will it make lots of roots so i can make a new big bush?!

Can you tell i'm desperate for a mile a minute bit of shrubbery, without spending lots of cash i don't have?!

This is a boring reply but it depends on the bush - some you can grow from softwood cuttings in late spring / early summer and others from hardwood cuttings in Autumn. If you google it you can find out which. In both cases you plant the cuttings in compost (indoors for softwood and outdoors for hardwood I think). If the bush produces flowers you can sometimes grow new ones from seed or some shrubs will grow a new shrub if you pin down a branch into the soil with a brick or stone and where the branch touches the soil it will root. If you're not fussy about shrubbery then Buddleija's good as it grows really quickly - planting a small shrub now will result in a big spreading shrub by July and it self-seeds everywhere creating loads of new ones. Or you could shove some shrub in water and see what happens!

Trying to root a big cutting will not mean you will get a head start to a bigger plant. The lack of roots will not be able to support the size of the big cutting. For info on how to do cuttings, I think the RHS website is quite good in the advice section. here.

As Whistlejacket said, do different cuttings for different bushes and at different times of year. RHS again explains.

Also, don't put it in water, put in compost or rooting jelly.