The trouble is that the Government has got into a bit of a bind over this flexible return to work malarky.
Read through this - which comes from the Government's own website.
http://www.direct.gov...leworking/DG_10029491
The gist is that you have a right to apply for it, but the employer has a reasonable right to refuse it and give you reasons. A 'difficult' employer will seek to find reasons why not.
What's different about your case is that you started a part-time role and now they've changed all that. A pity in hindsight that no-one put anything in writing, however you might be able to use it to your advantage. Having read through, think about what reasons they are giving for not continuing with the arrangement.