This was required for quite some time, to prevent it sticking or bubbling over, so be prepared to focus solely on this whilst you make it. Well worth the effort though. The cooking and cooling process seem to intensify the flavour of these wonderful flowers ten fold. Make sure you harvest from more pleasantly scented Elders.
I bought the River Cottage Hedgerow Handbook which this recipe is from recently. However it is also posted online by the author, within the River Cottage website. So no need for me to hold back, have a look at the link and get picking and brewing those flowers!