Amongst the replies to my original some of which were more funny than informative (not complaining) and given some had found useless products I thought this humble DIY'er might tell what I found. When in doubt I asked British Ceramic Tile for their advice and they sold me a product from Tileasy - Residual Grout Remover. It has 10% Hydrochloric acid and it works perfectly. I diluted it 1:2 parts water - no more residual grout. Given the relatively high concentration of acid the usual precautions should be taken. Thanks everyone.