Roof Leaks

I have had a leak on my bedroom wall and a leak on the side of the chimney breast in the attic for a while. I put an ad on MyBuilder in Sept 2018 and got a few quoted around £900. Another man rang me, rather than putting a quote, undercut the rest and i agreed on £600. My son told me (I was at work) that the main guy was holding the ladder while his his accomplice was up on the roof - 1.5 hours and they had gone. I agreed on bank transfer. When i got home I noticed from the attic window the gutters on the roof over the en-suite bathroom below still had grass and debris. I txt the main fella and he was very apologetic. Came back later on and his mate cleaned it. He begged me for a positive feedback on the site. I then transferred all the money. We had quite a dry spell for a while so I couldn't see the result from the job. A month or two later when it rained, it continued to leak. I had quite a few problems to deal with and didn't contact the roofer until May this year. After his inspection, he txt me that "the front pointing on the corner of the chimney has cracked and come away hence the leak in the attic.The major work is the gutter and step flashing, the step flashing under trays have deteriorate due to too much water from t he guttering - which is causing the major leak." He proposed to "replace the lead under tray, seal all lead, fibreglass lead and under tray and redirect guttering" for £450. I challenged him that that is what I initially paid him to sort initially. He replied that the bedroom leak was "a separate job and they didn't do that lead work last time and unfortunately it's quite bad. We paid more attention to the roof and didn't notice it because the roof was in such a bad state" He also said: "As with all our repair works there are no guarantees". The invoice he eventually emailed me last year listed: replacement slates; lead sealed and pinned in place; ridges re-pointed; velux window sealed; gutters cleaned, slates sealed between gaps. Total £600. I since put the same job on MyBuilder again and only had 1 recommended builder who pulled out; I found another, who had a look from my bedroom and attic windows, telling me about a "split in the leadwork of the valley between the two adjacent roofs; slates needed changing, cement the missing tiles and repointing the chimney. He asked for £750 (chimney) and £1100 for the valley and changing/lifting surrounding slates. Also he wasn't sure if his cherry-picker would fit and said I could potentially have to pay another £200 for scaffolding. Another recommended roofer stood from the alleyway looking to diagnose the problems who quoted £450 for "a few new slates; a few placing back, ridges pointing (and anything else while up on t he roof - at no charge). I have lost confidence in how to sort this nightmare with so many different views. I am not sure if any aspect could be covered by Buildings insurance. I was also advised to consider consulting Trading Standards. Any advice would be greatly appreciated.