We carefully cut out our old windows leaving a very small amount around the edge. They were faded and leaking/split.
Then bought some clear plastic sheet of same quality and stuck new in with rubber glue onto the small edge strip we left.
No leaks and you can see out again
Bit of a diy job but you can't see from the outside and once the glue set it was all trimmed up inside and it was quite cheap.
Took a couple of hours, plastic was from ebay and glue from B&Q.