Copied from a thread on Mumsnet:
Bit of background; I was a PIP Disability Assessor for a while this year, until I decided that I couldn't work within the system with a good conscience, and left. I witnessed DA's who were trying their best, mostly, to help people with their claims. They (and I) were hamstrung both by our immediate employers, and a system (whether by accident or design) that discriminated against claimants with certain conditions. In the spirit of the release of 'I, Daniel Blake', which isn't about PIP but other failures of the benefit system, I've decided to give some general advice to those looking to claim PIP.
Firstly, let's look at initially making the claim. I would say the most vital point is this; PROVIDE EVIDENCE. The way that the claim form/questionnaire is worded implies that the DWP/ATOS/CAPITA or whoever will chase letters from GPS, specialists etc to back up your claim for you. In my experience, this doesn't happen. The onus is on you to provide this information, although it would be very easy to assume the opposite. Some photocopies of a recent prescription, letters detailing diagnoses and appointment dates, and evidence of secondary care involvement (especially people with mental health conditions) provide good evidence; if you haven't provided anything, then the DA has to go mostly on that short assessment with you. You may be horribly unwell or disabled for 5 days a week, if you're having a 'good day' on the assessment however, then your claim is in trouble. Evidence, Evidence, Evidence; I can't stress that enough. Don't let the DWP have the easy option; the decision they make is heavily dependent on the report written by the health professional that assesses you. The decision maker at the DWP has no medical qualifications of any sort.
If you have disabilities that cause you to have problems getting out (especially severe MH conditions), ask for a home assessment. The way the system is, if you have a MH condition involving severe anxiety or depression, or agoraphobia; then you attend a clinic appointment, it can be used as evidence that you aren't as bad as you claim that you are. The reasoning is that you can't be that ill if you can get to a clinic appointment. It's profoundly unfair, and a large part of why I left. Once again; evidence is so important.
Don't assume that the health professional that sees you knows much about your condition. In a working and well-designed system, those with certain conditions would be given appointments with HP's with a specialism in that area. It ain't so. You could get a general or MH nurse, an Occupational Therapist, a Physio or a Paramedic. It's completely random. If you can bring along a health professional of your own, then do. I'd advise anyone against attending by themselves. That's not because I don't trust the DA's; the vast majority are good people. It's just easy to forget things, and people with severe MH problems or learning disabilities/Autism really need an advocate of some sort.
If you're asked to do a set of 12 exercises (it's called the Musculoskeletal exam, or MSK), be sensible! I had people trying to please me by trying to do these despite evident pain; tell the DA why it would be difficult for you to do it. If you cannot do it, just do what you can and explain why. Completing those ridiculous exercises at personal cost, especially pain, will count against you. Don't make stuff up to help your claim (they'll see straight through it, and assume that you're dishonest), but don't put yourself at risk of harm by doing something that you can't. The DA has to take your safety into consideration when assessing what you can and can't do. Once again, evidence is key. If you need aids or adaptations to walk, or need to wear wrist supports etc, then bring/wear them. Help the DA to help you. They normally will.
Talking of assessments, if you report any MH conditions at all, then the DA will be observing you throughout the assessment for a Mental State Examination (MSE). In my opinion, this is even more badly designed that the MSK. A HP, who may or may not have any MH experience at all, will continually assess your mood, behaviour, anxiety etc throughout the assessment. It's completely subjective. If you have a serious MH condition, if you are in pain, if you are distressed or anxious then let it show to the DA. The stiff upper lip is the worst possible approach.
We were told that those 45 minutes were key. If a consultant wrote that you couldn't do something, then you did it in that 45 minute assessment (good day or not) then the weight of evidence would be on the assessment.
My opinion, for what it's worth, is that PIP discriminates against those with learning disabilities or MH conditions. Seriously. It's appalling. If you are turned down, then PLEASE appeal. You'd be surprised how many obtain PIP through appeal; it's often through providing further evidence. Don't give the DWP the chance to deny your rightful claim in the first place; provide that evidence when you first fill in the forms.