Sorry to hear about this. Have you been given a reason why they declined it (I'd ask for one in writing if not) - unfortunately requests for workload adjustments are often the hardest for businesses to accommodate, as unlike say providing some equipment or making adjustments to the physical premises, it's hard to know exactly what the cost/impact of changing someone's job will be in the long term, it can mean impacts on the wider team/colleagues/clients etc. The law does allow businesses to refuse adjustments if they would have an unreasonable detrimental impact on the business - however they are not allowed to say no because they think you are 'not disabled enough' (so long as you do meet the legal definition of disability of course) or similar reasons, hence why I would want to know their reasons.
When you say OH have been supportive, have they actually written a proper formal report to your manager saying you do meet the definition of disability as per the Equality Act (as you mention yours is a non-obvious condition), triggering the legal requirement to make reasonable adjustments, and the change in workload is an adjustment you need because of your disability? Or have they just generally told you it's a good idea to reduce your workload? If the latter I'd go back to them and get them to write it up as a formal recommendation. Your manager might still say no but if they are not aware of the legal position and have not sought HR advice etc. it might at least get him/her to do so!
Also, have you thought about whether there is anything else the company could do to help you/compromises that could be made - how does what you are asking for in terms of workload/work pattern vary from what you have now? Could you see if 70:30 might be possible if not 60:40. What work would you be doing in the office based time, is there enough of it to justify you being in the office more and what will happen to the fieldwork you will be dropping, will colleagues have to pick it up? Is there anything that can be done to mitigate the impact on them? Have you considered asking to drop your hours rather than stay on same hours but with a workload change? Finally could you suggest a trial period if s/he is not happy to commit to a permanent change?
If you've tried all the above and the answer is still no, and you feel your manager's reasons are not fair/justified, then your next step would be to put in a formal grievance based on indirect disability discrimination. This is quite a hard and stressful thing to do though so if you can possibly sort it out directly with your manager then that would be better - are you in a trade union? If not then perhaps seek some advice from ACAS or you could speak to your HR informally to see if there's anything they can do?