1. What sort of system, Combi, vented hot water (not combi, cold water storage tank in loft, hot water cylinder in airing cupboard), unvented hot water (no cold water storage cistern), thermal store?
2. Shouldn't be expensive to fix, but depends on cause.
3. Is bath tap multi-turn or 1/4 turn?
4. Does hammer occur when turning tap on, turning tap off, or at all times when tap running?