I'm following up on the above. Without some further information from you as above, I am limited in what I can say on the matter but in the hope it is helpful nonetheless, I will provide you with the following broad answer. If you are able to kindly provide me with the above further information or if you have any further questions generally, I will be delighted to expand on the following - please just reply back to me in this case:
Notwithstanding the above, in general terms as you will be aware, you will have a statutory right to acquire the freehold and the aims of the upstairs owner involve acquiring the loft which will presumably add significant value to his property. If you do not join in with the purchase of the freehold, then as you say, the upstairs owner will have free rein to make decisions in relation to the freehold and also be able to grant whatever parts of the property which have not been demised to either review to himself for nil consideratiion.
By contrast, if you join in with the freehold, he will need your permission in respect of all substantive actions in relation to the freehold and you will be able to set out any terms you wish as a condition for allowing him to acquire the loft space or indeed simply refuse permission. There are no limits as regards ***** ***** you could demand as a condition for allowing him to acquire the loft space and therefore, potentially, though of course I do not suggest any form of guarantee, you could drive a hard bargain to recover either the entire cost or indeed even more potentially subject to commercial realities, of the costs off accquisition of the freehold.
Generally, acquisition of a share of the freehold will improve the value of your property but not necessarily by the amount you pay for the freehold. It may be worth taking advice from a local estate agent in this respect