Vaguely approaching it from a UK Patents Act point of view that is the foundation for most UK digital IP but IANAL (do get counsel): 1. If you worked on this i

Vaguely approaching it from a UK Patents Act point of view that is the foundation for most UK digital IP but IANAL (do get counsel): 1. If you worked on this in work hours, your employer may have a claim to the IP. 2. If you worked on this on work equipment, your employer may have a claim to the IP. 3. If you worked on this using employer resources (including from the office), your employer may have a claim to the IP. 4. If none of the above are true and you were asked to work on this by your employer and you were not previously working on it, your employer may have a claim to the IP. 5. If the IP has a degree of similarity with IP by your employer and was started (or contributed to significantly) after your employment started, your employer may have a claim to the IP. 6. If all of the above are not true, your employer does not have a claim to the IP. Any declarations or contracts that you enter into should reflect IP touched upon by #1-5 of the above and explicitly state whether or not your employer will declare an interest, no reasonable contract should preclude you from work you undertake covered by #6 (in your own time, your own resources, not asked for by work, unrelated to work and any future area that you know they may expand into, etc).