I currently work 3 days and can't/don't want to do any more. Employer is looking at how the dept can meet the needs of the organisation. I've been told this could mean redundancy for me. I'm guessing they might want to create a full time role. Where do I stand? Can they just make me redundant if this is the case?
If they need 37 hours and you currently do 21 or whatever, then they are allowed to explore how that business need can be met. They should offer you more hours, and if there is a prospect of your being made redundant, should consult with you about it, which would include considering any alternative suggestions you make about how to achieve what they need. Suggestions you might make include recruiting a job share rather than a new person, adjusting how tasks are done to enable you to do more in your 3 days, or a compromise of you doing 4 days for a trial period to see if that meets their needs.
But ultimately, assuming they consult properly and consider any suggestions you make, then if none of those suggestions are practicable, or you don't make any, then yes they could make you redundant.