There is no set figure for a percentage of your income. In this country lenders tend to offer a multiple of your income, say between 3 and 5 times, though this varies between lenders, quite significantly.
In France and Spain lenders look at your net monthly income (EG: after tax) and will offer a loan which gives a monthly repayment up to a set level of your income.
Many lenders in this country are now using a similar system based on affordability, so perhaps if your income is only �10,000 per annum 50% of your income would be a very high amount to lend as the remaining 50% wouldn't leave enough for household running costs, but if you earn �75,000 if you had to live on 50% of the income after the mortgage it wouldn;t be a problem.
Take some independant advice from a mortgage advisor near you as there are so many options it can be bewildering!
Hope this helps a bit! Dan