I should have posted this before with the info above but was in a bit of a rush at the time and just remembered it! This information came from a FOI request I recently filed with the British Ministry of Defence - don't know whether I asked the right questions (I'm a beginner!) or whether this adds much to the debate, but for anyone who's interested, here goes:
"You have requested evidence about the BAE Replica programme, believed to have run between 1994 and 1999.
The Testbed model continues to form an active part of the UK's signature control reduction programme and as such, there is very little detail that can be released at the moment. I have attached a press release which gives the general background to the programme and will attempt to answer your specific questions.
Question: Was this aircraft ever produced, even if just in the form of a prototype or technology demonstrator?
Answer: There was no series production of this design.
Question: How many mock-ups were produced, and what is their current whereabouts?
Answer: There was just one Testbed produced and it remains at BAES Warton where it was built. It should not be called a "mock-up", however. It is completely real as far as observing sensors are concerned, the externals being built with representative materials and methods of construction. Internally it is unrepresentative of aircraft in order to reduce costs of manufacture.
Question: Was Replica designed as a replacement for the Tornado GR4, or as a fighter aircraft?
Answer: The Replica programme was a joint research programme funded by MoD and BAES. It was never intended to lead to a production aircraft, but rather provide a focus for the UK's air vehicle signature control research. The Replica/Testbed concept emerged from an extensive series of studies into different aircraft configurations that addressed fighter, multi-role and strike aircraft. There was no Air Staff Target, but rather the aircraft was designed to a "sensible" set of performance requirements emerging from the Future Offensive Aircraft programme. The Replica/Testbed is very much a bomber rather than a fighter, with long range and large payload, although studies indicated that with internal weapons carriage and very clean aerodynamic design, it was able to outrun many contemporary fighters at low level.
Once the work on Testbed is completed, a much fuller version of the story will be able to be told. What can be said is that many of the technologies developed for Replica/Testbed are being applied across a wide range of current programmes."
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Hope that's of interest to people and if anyone wants a copy of the press release I was sent, feel free to PM me.