As you says might need acid, have searched during the day and so far it looks like phosphoric acid descaler is the one but don't know much about it yet and the other one I'm not sure of is hydrochloric acid based descaler.
Put loo paper on the glass then wet it with your spray descaler. The blue harpic bog one works and doesn't seem to attack anything too much.
The paper will give the elixir a better chance to do its job. If there's no effect at all I'd share masona's query - is the glass pitted now?
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local