I've just been on the PlusNet website, and for a broadband-only service they're quoting me £22.49pm with no contract, £19.99 with 12-month contract. Admittedly this is with unlimited usage, but there appears to be no other option.
If I opt for phone service as well, the cost is £25.95 for the first 12 months including line rental, on 12-month contract. This includes a promotional discount of £7.49pm.
I guess this ties in with your figure of £5.99pm for broadband.
I'm slightly tempted, but am a bit reluctant to change my phone service supplier.