From research the company of Martin Markham was formed in 1952 from the two companies of Markham Traction LTD and Martin Cultivator Company.
I have a 1947 advert for a Markham Traction trailer and also a 1958 advert for a Martin Markham trailer, so an advert before the merger and one after the companies merger. Both appear to be different trailers and yours is much like the 1958 advert for their Champion trailer.
There appears to be a lot of dealers for the trailers including: E.H.Knights & Sons LTD, Harleston. Rose Bros (Tamworth) LTD. Aylesbury Motor Co LTD. W.Harold Perry LTD, Whetstone. Hugh Craigs, Market Square, Ballyclare, Ireland.
Price for the 1958 trailer was £170 ex. works.
I’ve attached the images as thumbnails but larger images are now uploaded to the gallery, just click the images/links below:
Markham-Champion-Trailer-1958
Markham-trailer-drawbar-closeup
Markham-trailer-1947