Sum constant 52 previous cells in one column ignoring clear cells

Hi - First post so Hi everyone, hopefully attachment is present.

Using my example file Im trying to find a way of:

Summing column 'K' Cells each week in the column 'L' Cells for a Constant previous 52 weeks in the 'K' Cells

ie. If there are cells with no figures in column K the formula needs to jump back another cell so its still doing a sum on 52 figures. (Also Please note: There is a formula in the cells in column K, so things like isblank dont work as its see's something there).

Thanks so much in advance, ive used excel many years but this really has stumped me.