When purchased as 2 half boxsets, a 9 and 7 disc boxsets available at £17.99 each.
The Adventures Of Sherlock Holmes/The Return Of Sherlock Holmes 9 disc boxset
The Casebook Of Sherlock Holmes/The Memoirs Of Sherlock Holmes 7 disc boxset
I could not find a working discount code, the WT8747 code didn't work for me. 7.5% is available via quidco though.
HMV do sell a combined 16 disc boxset on its own but its currently £119.99!