We demonstrate a simple design for an in-fibre Fabry-Perot interferometer (FPI) equivalent, constructed by splicing together two ports of a 99:1 in-fibre beamsplitter to form a fibre ring resonator (FRR). A Peltier device is used to heat a length of the fibre, which scans the resonant frequency of the FPI in a controllable manner primarily by changing the refractive index of the fibre. This allows the device to be employed in applications where a standard Fabry-Perot cavity is used, in this case for laser mode monitoring. Our FRR has a measured finesse of 123(2) and is shown to be stable over long time periods. This provides an inexpensive and simple solution for applications with low performance requirements, and with relatively minor upgrades could be used for more exacting applications such as frequency locking.