I'm just posting this as an idea to other expectant mums complaining of back ache.
I bought a tens machine for £40 from Boots and it's been brilliant.
It's nto as strong as the ones you'd use in about, it has a choice of two or four pads.
I put it on if I wake up with back ache in the night or morning. Really helps me sleep or get back to sleep.
Recommend it highly - never thought of it during my first pregnancy.(I'm 25 weeks - had it since 21 weeks)