It very much depends on the size of the organisation you work for and whether the reduction of your hours would be financially viable. Under the Equality Act (previously the Disability Discrimination Act) a person is regarded as 'disabled' from the point of cancer diagnosis onwards and is covered by this act from thereon in - regardless of whether they are 'cured', in remission or undertaking treatment. You could ask for a reasonable adjustment to reduce your hours under the Equality Act. This would best be supported with Union rep and/or occupation health and/or your GP. If the company is large (or local government) and it won't cost them too much then this is possible. You could request this on a temporary basis first and this would further support your request to make this permanent at a later date (i.e. if company can cope with it for a temporary period then they may be most accepting of making it permanent).
I would suggest you request a return to work on a part-time basis first to see if the company/organization raises any objections.
Also the CAB is a very good organisation for this type of query. Here is the link to the Equality Commission website in respect of reasonable adjustments but do venture into other areas of this website at there is a lot of information on your rights and employer responsibility:
Hi lemon grove, I am no expert, but there are some people on here who seem quite knowledgeable, I have come across lots of threads discussing returning to work.
My first port of call would be the citizen's advice bureau, but if you have a union then speak to them, or your HR department? When I had issues with wanting to work part time, a few years ago when my children were small, I went on to the ACAS website which had some very good downloads on employee rights.
I hope you get the hours you want from your employer.
I've been off sick for 11 months now, and want to return to work on a part time basis, rather than full time. I'm not really interested in a phased return, as I never want to work full time again. Can I insist they change my hours?