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Thanks, redcrx for replying but it is worth my while to take up Sky on this offer because the HD box will act in exactly the same way as a Sky+ box does until we get an HD ready tv. You are only allowed one HD box using the offer!
The Sky HD rep was in the supermarket this afternoon and although I'll be spending an extra £9.75 per month for a service I won't receive it works out cheaper in the short term than forking out a huge amount for a second Sky+, either new from Sky or secondhand from Amazon then hoping for a local company to come and, hopefully, set it up. Sorted!