maclarencat has certainly indicated the gamble one takes with the replacement of windows.
We were very lucky to have had a local firm who did a really good job. Perhaps this was because we kept tabs on things as work progressed and made sure the workmen were on our side.( A few pounds in their pockets, along with tea, coffee and biscuits etc. works wonders.)
However, although we had a 20yr guarantee, the firm went bankrupt a few months after installing our windows.
You pays your money and takes your choice!!!.......Best wishes....Viv & Ron.