Try asking your pharmacist about separating the ingredients in Syndol into their generic equivalents. Syndol contains paracetamol, codeine, caffeine and doxylamine. It might well be the continued use of paracetamol that's causing your stomach problems, so dropping that from what you take (and possibly increasing the dosage of something else) might be worth a try.
However it's worth remembering that you're almost certainly NOT suffering from shingles itself. (It simply doesn't last that long). You've apparently been left with postherpetic neuralgia and it's that condition (rather than shingles) that you should be seeking to treat. See here for some suggested treatments:
http://www.nhs.uk/Conditions/postherpetic-neuralgia/Pages/Treatment.aspx