After establishment, heaths and heathers prefer pretty dry soil conditions. In fact, they are exceedingly drought tolerant and do not care for consistently moist soils. And they need full sun.

It's not a combo I'd advocate :-)

Soil doesn't have to be acidic for Hydrangea macrophylla. Anyway if you don't care about what is presumably a Calluna not looking at all naturalistic with big-leaf hydrangea - and the cultural conditions appear to suit both - then there may be no barriers to this combination working out.

"Soil doesn't have to be acidic for Hydrangea macrophylla"

It does if you want blue flowers :-)

Thank you gardengal48 and Embothrium It didn't feel right to me but sometimes you just get so tired of planting the same combinations!