I don't think there is a way of making inspections reliable. There are always going to be flaws. There will always be weak teachers and weak OFSTED inspectors whether they are agency or not. I don't even know what the answer is! Although I will say judging a school using a 'one word and number' criteria is not helpful and quite damaging.
My school was judged before the actual inspection, the inspector (a non-primary ex-headteacher inspecting a primary school) admitted to the governors that his visit (and a question he asked about the LA) WAS political; we were one of 15 schools in our county blitzed 'not so long ago'. That inspector was ruthless, asked questions after questions and didn't give staff time to reply to them, cutting them off mid-sentence to ask another question and made it clear from the onset that he was there to 'fail us'. We didn't, but we didn't come out of it with the fair 'grade' we deserved. He, I believe, was a weak inspector who relied on data (he admitted that had he arrived two months later sfter the KS2 results were released to schools, his judgement would probably have been different) rather than the judgement of what he saw in school to base his overall judgement.
We were all left bruised and battered with a complete loss of self esteem and confidence. Luckily we had a head who was driven to prove them wrong and the night she received the preliminary results, she set to work on the action plan. Even HMI couldn't understand 'what it was that we were doing wrong'. She made one visit and gave 'us' her number and said "call me if you have any problems or queries!?!?!"
Personally I would have liked us to object to the findings but there is little faith in the OFSTED regime that this would not have been effective and still would not have resulted in the fair grade we deserved.
Thankfully we have parents on our side and could see through the political agenda of OFSTED and the government.
Society isn't stupid. The flaws in OFSTED are beginning to show and it really won't be long before there is a complete, radical overhaul of the entire inspection system because even PARENTS are beginning to mistrust their judgements.
My local primary school that my son is about to start St, was 'outstanding' four years ago. Three years later they were judged to be 'inadequate' based wholly on data!!!!