I would say one lemon is a bit light, especially if not using Citric acid.
White grape juice is a good addition if you can find it.
Also, my usual method differs in that after trimming elderflowers I cover with boiling water and add a crushed campden tablet and leave for 24 hours to kill off wild yeast.
I also use 1 tsp pectolase per gallon
I've found it useful to use a grain bag to make straining at the end of fermentation easier.
The BBC doesn't mention taking an gravity reading which helps for telling how fermentation has progressed.
Batch I made this year showed little sign of fermentation but a sample showed it was progressing fine and I transferred to demi John's at .998 from OG of 1.084
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Edit: My recipe here
Tanglefoot's fruity fermentations