no html signature in replies/forwarding.

I've had to change my pc and have installed Thunderbird again on the new one. All up and running, getting, sending emails etc. I have created an html signature block that includes text and an image. This works fine when writing an email, the text and image appearing as expected in the email. However if I do a reply or an forward in response to a received email, there is no signature block. I have been into my Account Settings via Tools -> Account Settings -> Composition and Addressing and have ticks against both 'Include signatures for replies' and 'Include signature for forwards'. Have I missed something? The is all worked fine on the other pc so at a bit of a loss.