It may depend on what you mean by a masters in a Psychology related area and what you want to do in terms of future career. If you're staying with nursing, then you don't need the Master's now (you'll need it later on for band 7/8 level, but by then your trust should help you get on). If you're looking to transition to Psychology then the honest answer is that it is probably impossible in your current circumstances.
If you're looking to gain BPS accreditation (i.e. conversion masters) then they will have a minimum criteria and the lowest I can see that being is 2:2; though contacting courses directly would be advisable. I'm not sure what the Open University would want? If you're looking for a general Psychology masters, then it may depend on how closely aligned it is with the BPS; for example, a Forensic Psychology masters will require you to have BPS accreditation and a 2:1, whereas others will have lower requirements.
If you can't get the conversion or masters in this way, then perhaps completing a Psychology BSc? It's a more expensive and longer route, but if you're set on it, then it is probably your only real option.