Downhole tool

1. An activating device configured to be insertable into tubing at a surface location and to pass through the tubing to land on a seat in the tubing at a downhole location and experience landing forces on engaging the tubing seat, the activating device comprising:

an activation profile configurable to be maintained at a larger diameter than the tubing seat,

the activation profile including a radially movable member supported in said larger diameter configuration by a support mechanism having at least two parts in a release configuration, the parts of the support mechanism being relatively movable to remove radial support for the radially movable member and allow the radially movable member to radially retract to define a release diameter, and

upon the activating device descending through the tubing, the activation profile engaging the tubing seat.

2. The device of claim 1 , wherein with the profile re-configured the device is adapted to pass through the seat.

3. The device of claim 1 , further comprising a seal for providing a fluid-pressure seal between the device and surrounding tubing.

4. The device of claim 1 , in combination with a pressure sensitive downhole device, whereby an increase in pressure above or across the activating device at least one of activates and actuates the pressure sensitive device.

5. The device of claim 4 , wherein the pressure sensitive device is configured to utilise a differential pressure between an interior of the tubing and an exterior of the tubing or a differential pressure across the activating device.

6. The device of claim 1 , wherein the device is configured such that applying a release force to the support mechanism re-configures the profile.

7. The device of claim 6 , wherein the device is configured to permit application of the release force from above the device.

8. The device of claim 1 , wherein the device is configured to maintain the profile at said larger diameter until application of a release force above a predetermined magnitude to the support mechanism.

9. The device of claim 8 , wherein the device is configured such that applying a mechanical release force to the support mechanism re-configures the profile.

10. The device of claim 1 , further comprising providing the support mechanism with a release portion configured to be exposed to fluid pressure acting above the activating device, the cross section of the release portion being of a smaller cross sectional area relative to an area of the body exposed to fluid pressure acting above the activating device.

11. The device of claim 1 , further comprising providing the activating device in a tubular form whereby the activating device does not completely occlude the tubing on landing on the seat.

12. The device of claim 1 , further comprising a deformable portion defining a larger diameter than the tubing seat for engaging a surface of the tubing prior to the radially retractable member engaging the seat.

13. A downhole method comprising:

introducing an activating device having an activation profile including a radially movable member into tubing at a surface location;

configuring a support mechanism to radially support the radially movable member to maintain a larger diameter than a seat provided at a downhole location in the tubing;

passing the activating device through the tubing from the surface location to the downhole location to land the activating device on the seat, whereby the activating device experiences landing forces on engaging the tubing seat; and subsequently

relatively moving parts of the support mechanism to remove radial support from the radially movable member such that the radially movable member radially retracts and the activating device passes through the seat.

14. The method of claim 13 , further comprising increasing the pressure in the tubing above or across the device.

15. The method of claim 14 , further comprising utilising the increased pressure above or across the activating device to at least one of activate and actuate a downhole device.

16. The method of claim 14 , further comprising utilising a differential pressure between the tubing and an exterior of the tubing or across the activating device to perform a function.

17. The method of claim 13 , comprising reconfiguring the profile by applying a release force to the support mechanism.

18. The method of claim 17 , wherein the release force is applied from above the activating device.

19. The method of claim 17 , comprising maintaining the profile at said larger diameter until application of a release force above a predetermined magnitude.

20. The method of claim 17 , wherein the release force includes a mechanical force.

21. The method of claim 17 , wherein the release force includes a fluid pressure force.