I just heard on Gardeners question time that Gooseberries love potash. Is nettle tea - rotten nettles in water - a good source of potash.
Not sure about the nettles but did see in Wyevale's this morning they are selling it for a £1 a box in their sale.
Or you could always have a bonfire and use the ash from that. But sure someone will be along soon who knows about nettles.
Sinbad
if going to use ash from bonfire as potash source you should keep this in mind.
1. ash is very strong potash source so use it sparingly
2. a manufactured fertilizer either organic or chemical will give more control of fertilizer release and concentration.
Or comfrey ? ;)