If there isn't a contract that commits either of you to anything then I think it is a separate transaction. The risk is that HMRC decides that a transaction was deliberately split to avoid the tax, which I'm sure you going to be honest about - ie it's not a firm plan is it really, more you're just toying with the idea should on the off chance it come up for sale in the future.
Ref:
http://www.hmrc.gov.uk/s...te/linked-transfers.htm
Am I correct is assuming your offer to buy the land along with the propery was unfortunately turned down as the owner didn't want to sell it (I'm sure their letter to that effect is carefully filed isn't it).
If the land is offered on the open market at some later time such that anyone could buy it and no evidence exists that the transaction had been linked then I would think HMRC would have a tuff time proving it was.