The problem is almost certainly that the wall is not waterproof and has not been tanked. Replacing the bath trim wont stop the leak. All the tiles above the bath have to come off. If the wall is perfectly flat and dry then tank it. If not use Marmox (or similar waterproof boards) - you can screw 6mm boards to wall and then tile over. If you have not used waterproof boards before - check manufacturer specs - they usually require screw fixing with plastic washers, screwed in rawl plugs and sealed boards joints and screw holes with Silicon. To ensure boards fix perfectly flat Marmox also recommend tile adhesive spread on wall with 6mm trowel so that board edges are level.