Good information here, including the freephone number for ACAS who will give you the answers to your specific questions:
http://www.direct.gov.uk/en/Employment/Employe es/RedundancyAndLeavingYourJob/DG_10026616
Any 'new job' with the same employer in the circumstances you describe must have a trial period (and other considerations) of a minimum four weeks.
If the job is refused before the end of the trial period then redundancy will be effective - if it is turned down for a good reason. This includes a change of salary.
Normally, if the new job has less pay, the employee has 'protected pay' for a period. This can be, for example, no cost of living rise until the new job has reached the same pay as he is getting.
More good information here:
http://tinyurl.com/5loqlv