Really ? Triton showers always come out one of the best in reviews. You say to replace the PRD, I am not sure if I need the t shaped thing with the disc seals or just the disc seals. The latter you can get 6 for £5.99 on AmazonReplace the PRD. You can get one from Triton or showerdoc.com or other places.
Throw the filter shower head in the bin.
Make sure the original shower head is descaled and put it back on.
Triton showers aren't great to be honest but its not their fault if you fit an aftermarket part that isn't suitable.
You say to throw away the shower head but this did cost almost £35 and again it has great reviews which I find strange. I'll sell it on Marketplace but I still have the problem of chlorinated water coming out of the shower which is terrible in my area.