diameter there is thus created a annular section of increased tubing diameter 36 at the tubing end section 20 a as shown in fig2 which provides an interference fit with the surrounding liner 14 the sealing bands 22 being deformed to form a fluid tight seal between the expanded tubing 36 and the liner 14 the hard material in the grip bands 23 also assists in keying the tubing section 36 to the liner 14 there may be a degree of elastic and even plastic deformation of the liner 14 which will serve to provide a more secure location for the straddle 10 following creation of the annular extension 36 the pressure in the tool string 26 is reduced such that the rollers 32 may retract the expander 28 is then advanced towards the lower end of the straddle 10 and engages a stop 38 provided on the lower end of the tubing 20 the pressure in the tool string is then increased once more to actuate the rollers 32 and the expander 28 is rotated to create a second annular section of increased diameter 40 the expander 28 may then be deactivated and retrieved from the bore leaving the straddle 10 locked in place in the bore and serving to isolate the formation 16 from the bore to remove the straddle 10 the locking and sealing sections 36 40 are milled out and the remaining section of tubing then removed in other embodiments the increased diameter sections 36 40 may be formed simultaneously by provision of two expanders located one at either end of.