I agree with you that this is the JH mark of Howell & Co and that it is a London hallmark. I also think that the date letter is "n". For me, from the evidence of the little flourishes on the side of the letter, it can only by the 1928 "n" as this was the style of the letters in the 1916-1935 date letter cycle.
Phil
PS: after looking at it a bit longer I wonder, from the relative height of the date letter compared with the whole punch, whether it might be "h" for 1923:
1928:
![Image](http://www.silvermakersmarks.co.uk/Dates/London/1928n.gif)
1923:
![Image](http://www.silvermakersmarks.co.uk/Dates/London/1923h.gif)