Just heard back from the Sofitel. Since I'm flying into and out of T5, I'll probably try this out in a few weeks when I'm there. Note that gym hours are 24h/day.

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The spa opening hours are:
Monday-Friday 08.00 - 21.00
Saturday - Sunday 09.00 - 21.00
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For non hotel guest there is a charge of £40.00 per person.for full use of the spa this.includes the hydro.suite (steam room, sauna and Jacuzzi) also gym access.
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For non hotel guest who wish to use gym and shower only the charge is £30.00 per person.
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If you have a treatement in value of £55 the spa facilities are complementary.