Re: hi--i just want to spend 25pounds and recieve the 75 promtion
Google will aggregate your daily limits to get you spending your total in a month - so if you want to spend just £25 you need to set a daily budget of 83p...
...but that's unikely to get you any traffic to speak of - so... you can set an end date to your campaign - set it for 5 days, say, and then set a daily limit of £5.
I don't want to be rude, but if you are serious about testing AdWords, you probably want to think about allocating a little more budget to the test. It's a kind of reap what you sow thing - if you are not really committed to testing - and that doesn't mean spending a fortune - far from it - you're unilkey to be satisfied with the results.
You need to get your head around the idea that Adwords is an advertising medium - you wouldn't expect to run the cheapest classified ad in your local paper (which would probably cost around the £25) - and run it once, to see if the medium could work for your business.
It's kind of unfair to hold Google to standards that seem ridiculous in any other advertising arena.