Question Author
I have just received a reply from the BBC. I think this is the best result that could be expected. Here is the text of the message:-
Thank you for your query regarding 'Mastermind', and specifically, the question regarding the chemical symbol for Thallium.
Due to an error on our part, this was wrongly given as Ti (which is actually the symbol for titanium) rather than Tl. In the heat of the moment, John Humphrys mistook a lowercase 'L' (l) for an uppercase I (I). However it was entirely the responsibility of the production team to spot this potential pitfall before the recording took place and to have picked up on the error during the actual recording.
Fortunately, this error didn't affect the result of the contest as the contender in question lost by two clear points. However, we obviously we take any mistakes very seriously on 'Mastermind', and we will do our utmost to ensure that it is not repeated. We will also be offering the contender in question a full apology and the opportunity to come back on the next series of 'Mastermind'.
Thank you for bringing this to our attention; I hope it did not interfere with your enjoyment of the programme.
Yours faithfully
Jon Kelly
Series Producer Mastermind