As you are probably already aware, the Mk1 & Mk2 are probably the rarest of the seven Titan variants and differ significantly in appearance to the later Mk3 to Mk7 models. The main difference between the Mk1 and the Mk2 was the cocking method with the Mk1 being cocked by pulling the muzzle plunger forward and the Mk 2 by pushing in a rod from the breech end as with all the later variants. I believe that there are only around half a dozen Mk1s known to collectors and even fewer Mk2s which will consequently be reflected in the asking price for such a rarity. You will need to carefully check it out for originality, functionality and cosmetic condition, but I would have thought somewhere between £1000 - £1500 to be in the right ball park for an example in good condition.
If you PM me I would be happy to let you have my email address so that you could email me some pictures which I could upload to the forum on your behalf.
Brian