The HR21 was rather large as compared with the H2540! And it was Marconi’s second-line point-to-point receiver, below the HR93, although I think fairly close in performance. I imagine that the H2540 equalled or bettered the HR93.
Re the H2900, in hindsight one might say that it was something of a “concept” receiver, with the H2540 the production realization of that concept, as refined by market response, intervening technology developments, etc.
The H2540 employed the same synthesizer as used in the H1540 transmitter drive, the latter being described in Communication & Broadcasting Volume 3, No.1, 1976 Summer, p.13ff. According to an article
“Versatility in h.f. systems” in Communication & Broadcasting Volume 4, No.2, 1978 Spring p,16ff, that whole product line was referred to as “MFT”, Marconi Fast Tuning, and does appear to have been the successor to the MST product line.
The next step was the incorporation of microprocessor control, in what was called the MFT2 range, MFT 2nd generation. This was described in an article
“MFT2 h.f. systems – synthesized drive and receiver” in Communication & Broadcasting Volume 6, No.2, 1981 March p.19ff. The receiver was the H2541, which appears to have been an updated H2540, retaining the same basic layout and IFs.
Cheers,