reporting back - changing from plus-net adsl2+ unlimited servicing - finally went for
John lewis (who themsleves sub-contract to plusnet) . I only needed adsl2+
John Lewis unlimited adsl2+, 12month with £40 cash-back effectively £17.60 p/m
versus re-contracting with plusnet directly £25.50
BT would have been cheaper £10.80 p/m - however will probably only need 6 months for which the BT ends up costing nearly £200 total versus JohnLewis £110.
[BT have punitive/crazy early cancellation fees £25p/m fee + router £65]