First urg question [so be gentle ]...

I really hope this doesn't sound as daft as it might first appear but...

I've got 4 small Acers [my own little Acer glade, for anyone who knows
the wonders of Westonbirt Arboretum. ] in pots on the dappled, shady
side of the garden. They're all doing quite well - albeit showing the
signs of a dry and warm wind summer, with some of the tips going a bit
crinkly - but I have a question about watering:

All the Acers are planted in an ericaceous compost [and top dressed with
a decorative gravel/slate mulch to help retain the moisture], and this
year I added a water butt for these watering purposes. However, this
summer has been so dry that I haven't really had enough rainwater to use
in these pots. But I've also been mindful of the fact that tap water
will effectively wash all the acid from the compost, so the question is
essentially this:

Is it best [assuming the Acers don't look like they're about to keel
over and die, obviously] to keep them relatively low on water and *only*
water with rainwater from the butt when available, or isn't the
ericaceous such an issue with Acers and I would actually be better off
keeping them well watered with tap water. Thanks.

--
Flower Bobdew
South Facing Garden
South West: UK