I have a 2006 RRS and all the doors and boot lock (althought the horn sounds) but on the dash it says "TAILGATE OPEN"
the boot seems to lock fine !! doe sit have a sensor in the boot ?

Interesting, had "Tailgate open" warning on dash this morning, but tailgate was closed. I opened and closed tailgate and same message appeared. On further investigation I discovered a shoulder strap off a tool bag was lying where the tailgate closes, it didn't stop it from closing but obviously didn't prevent the vehicle from picking up that the tailgate wasn't fully closed.

It looked closed and pulled closed as usual so maybe there is a sensor and the strap stopped it from being totally closed. RRS TDV8 MY2010 Stornoway Grey
Full Exterior Autobiography Factory Upgrade
Piano Black Interior Trim inc Steering Wheel
Ivory Premium Perforated Leather
22" 5 spoke "Style 7" Diamond Turned Alloys
Towing Pack
Diesel Tuning Box
Privacy Glass
Side Steps

Best vehicle I've ever driven.

Current garage also includes;
Mercedes 320 CDI CLS
TVR Tuscan S

yeah I thought there might be somehting trapped at first but there is nothing in the boot !!! frustrating as no reason why it would say TAILGATE OPEN

Had this myself with the horn sounding but it wasn't the tailgate. I suggest you open/close all doors. Start engine and switch off and then try and set the alarm again. I found it reset the condition the time I had it. TDV8 HSE, Remap (+45 BHP + 100 Nm), Taylored sport exhaust system by Pipe Werx
20" HST alloys, Privacy glass, Side steps, Xenon Adaptive Lighting, Adaptive cruise control

without sounding daft have to tried opening and closing(slamming) both parts of the tailgate see if that fixes it only the dead truely know peace,
Bootneck its not a job its a way of life!

The earlier sports have been known to have rusty faulty connectors for the tailgate perhaps it might be worth checking all connections.

I suffer with this occasionally. The issue is that the design of the closure mechanism is that you should NOT need to slam the boot but just get it to latch and the boot should then pull itself closed. You will be able to hear a whirring sound as the little electric motor pulls it tight shut.

Fixes:
Short term fix is to pull on the boot as if trying to open it once it has latched on the first click to encourage the motor to kick in.

Longer term, check the sensors by the latch on the floor of the boot area and tailgate for corrosion and also there is a rubber wedge on each side of the tailgate which are designed to not let the boot close to far. On mine these had gradually moved and were allowing the boot to close too much on first latch. Loosen the screw, adjust wedge, tighten screw. Job done  TDV8 now with extra grin factor thanks to Alive Tunning: Gone but not forgotten