Okay, so does this mean you have done a mandatory reconsideration (MR); had it refused and have now appealed or by appeal do you really mean MR?
MRs have a low (17% but possibly rising) success rate but you can largely do an MR yourself as you don't need new medical or other evidence. Ideally for an appeal you need representation and you can look in your local area for Citizens Advice, a law centre, any other independent advice centre, perhaps the national charity which supports your specific health issue(s), or a local authority welfare rights advice service. Almost all of the above will provide their services free of charge. It's only exceptionally someone would need to use a service which charges. Be aware that some services which charge will tell you there are no free services in your area when that may be untrue. Certainly happens in Greater Manchester for example.
You can try something like https://advicelocal.uk but I'd be the first to admit it's very hit and miss. It's worth securing representation because, if you attend an oral hearing (and assuming you have a winnable case in the first place) your chances of success are much higher with a representative.
In terms of medical evidence for ESA the usefulness and necessity really depends on your specific health issues. You'll already have a fit note from your GP so your diagnosis should not be in dispute. Your GP may be able to add great value re: mental health but less so re: physical health. What do they know of how your conditions impact you outside of their consulting room or overnight for example? Their repeating what you tell them adds credibility but doesn't carry the weight of medical evidence because it arguably isn't.
Where evidence from a GP can be good is in terms of understanding the impact of being found fit itself on your health and whether you might fit the grounds to be treated as having exceptional circumstances despite not scoring 15 points. Direct them to that rather than fitness for work itself. Their fit note has already expressed their view on that.
What might be better is if you look at the ESA points scoring system and identify which points you think you ought to score by asking yourself whether you do each activity safely and repeatedly in a work context. Then think of a couple of examples of what happened the last times you tried that specific activity or something similar (some of the activities are not things we would necessarily ever do). That's going to be at least as helpful to your case as focusing on medical evidence. There's no legal requirement for medical evidence for any benefit claim beyond fit notes and this is often forgotten. However, you do need to have other evidence in place. Anecdotal evidence done in detail is excellent.
Hi Mike, thanks for your advice. MR refused, now having to take the matter to court. I was moved to UC 3 weeks ago, 2 weeks ago I was referred for a sanction for missing a telephone appointment with my work coach, I explained that I'd had no money for 2 weeks, that my mobile had malfunctioned and I simply had no income to get it repaired. This explanation was ignored, I've complained to my MP who is taking this up for me with them. I'm 59 years old, suffer from COPD and auto-immune disease, having been living from food banks for the past week, a really degrading experience. A further appointment with my work coach yesterday, I had to walk 4 miles to attend, for fear of a further sanction. The whole UC system is highly punitive and draconian. Got annoyed yesterday at Jobcentre, and now in fear of a further sanction..