It's hard to give you specific advice on this without seeing the various contracts of employment etc you had with the 2 different employers, and having more specific details of the work you did which you say was outside your job description. Clearly there will be a boundary between what you did off your own bat and what was required of you in the normal course of employment, but it's not an easy matter to define the boundary exactly. For instance your job description might say you are responsible for regulating expenditure on stationery, and you decide to build a spreadsheet in your spare time to help you. I think in most instances this would be seen as 'part' of the job. However if you were responsible for running the stationery, but in your spare time you devised an advertising campaign for the charity which was very successful in bringing in new donations, that might well be seen as outside your job description. As I think you are already aware, you would own the copyright in any creative work that you did wholly outside your employment, subject to any contractual limitations.
I suspect that while you were with the charity you either did not have a contract or if so, it was possibly generic, and that the general attitude amongst the staff was more relaxed about who did what. On the other hand public bodies such as local councils can be remarkably hidebound and dominated by job descriptions. The TUPE regulations
http://www.bis.gov.uk/files/file20761.pdf say that employees should transfer to the new organisation under the same terms and conditons (except for occupational pension rights) as their old employment, and that if it is necessary to vary these terms, the employee must agree to them. So unless you agreed to the Council's intellectual property policy (assuming it was different to the policy of the Charity) the application of the council's rules now may be judged unfair. The Council are not allowed to vary your terms or conditions solely to bring you in line with those of the existing workforce of the council. And if your original terms and conditions (at the Charity) lead you to believe your extra work was outside your employment terms, I don't think the Council can apply their much more limiting rules retrospectively to what happened while you were working for the charity.
Clearly a lot depends on whether you are still employed by the council. If not. then they would have a very hard time trying to compel you to hand over the work; I assume therefore that you are still employed by them in some capacity and so you might be well advised to go to your union if you are a member, or the Citizen's Advice Bureau if not. There are lawyers who will advise your but at a cost, so I would try the union/CAB route first. If the council should threaten you with dismissal if you fail to hand over the work, you would probably have grounds to take them to an Employment Tribunal for unfair disnissal.
Since this is not really, at heart, a question of copyright, but rather one of employment contracts, I'm sorry not to be able to help you more.
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