Business Premises Renovation Allowance

If we spend on converting a disused commercial building and brought it back into use, assuming everything qualifies for BPRA, when can I claim the initial allowance if say

a) expenditure incurred during 2011/12
b) trade commences during 2012/13

Am I right in thinking that prior to commencing the trade the expenses are "pre-trade expenses" and so are treated as being incurred on first day of trade? So initial allowance is claimed in 2012/13 and not 2011/12.