OP reporting back with an update, and wanting more advice as I am disappointed in how this has been handled. Will be long, sorry 
TIA
First another AIBU. AIBU to expect the teacher to at least have the courtesy to reply to my email, sent Friday 10th? Given that I was polite and tactful and alerted her to her error before she dug herself into an even bigger hole and made herself look foolish? All it needed to be was "Thank you for the email, silly me, of course your DD is correct I will make sure I tell the class asap" . She could even have saved face by saying she had realised her error and was already making plans to correct it. I did not want gushing thanks or a public acknowledgement (DD would have been embarrassed if for example this was mentioned in class)
DD had the teacher again yesterday - she only has her 3 times a fortnight so this was the first time she had seen the class since the first 2 lessons after which I sent the email. Their other English teacher however had in the interim mentioned the correct monarch & time period and that Napoleon/nelson/Wellington were important figures at the time. Possibly a coincidence due to the part of the book they were covering, and not necessarily triggered by my email, though I do consider this may have been part of the setting things straight within the department. The other teacher's reference to Victorian was not mentioned/corrected at all though.
Anyway, yesterday the teacher was all "I don't know what I was thinking I realised as soon as you left last lesson that I had been saying Victorian, I was confused with the book I had been studying with my class the period before " Now, I do not buy this at all. Firstly the error was made in 2 lessons on 2 days. So she made the same "mistake" twice in a row? Once would have been bad enough, and you would make damn sure you corrected yourself immediately in the next lesson, no? Now, again I am not expecting her to publicly point out that someone's mum has emailed in and whoops teacher is wrong. But I really think she has done her self no favours in coming across as dishonest. What are DD and her friends to think? They know I emailed! Are they to think That teachers do not admit mistakes, do not acknowledge when pointed out? Do not thank people who point things out? How are they supposed to respect her when she can't even be courteous and honest? I dread to think how she would have handled this if DD had had the nerve to immediately query this in the lesson. Certainly not well done have 500 house points as someone suggested earlier. Don't think DD will be inclined to do so in future either now.
so what to do? I really do feel it is not on to not reply to my email and not acknowledge DD being correct. However, if I escalate things, given teacher has obviously not appreciated my intervention, I do not want her holding a grudge and making things tricky for DD.