The voluntary choice to convert from the TiVo-platform (which drives TiVo & V6 boxes) to the 360 platform, is completely separate from any multiroom installation. For any TiVo/V6 customer who is happy with their equipment, there is no requirement to migrate to the newer platform.
The only difference is that you've mentioned mini-boxes - these only exist in 360-land and don't have hard drives. If you're on the TiVo-platform, all additional boxes are fully functional V6 with hard drives.
You'll end up calling into VM to arrange a multiroom installation - the additional box will be somewhere in the £7-£10/month range, but that depends on your package and whether a newer bundle is more cost-effective. If you don't have a co-ax install point where you want the new box, then it'll be a manned installation. And be fully prepared for a sales pitch for 360 when you call, just politely decline if you don't want it.
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