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Recent ALMA observations identified one of the most massive star-formingcores yet observed in the Milky Way; SDC335-MM1, within the infrared dark cloudSDC335.579-0.292. Along with an accompanying core MM2, SDC335 appears to be inthe early stages of its star formation process. In this paper we aim toconstrain the properties of the stars forming within these two massivemillimetre sources. Observations of SDC335 at 6, 8, 23 and 25GHz were made withthe ATCA. We report the results of these continuum measurements, which combinedwith archival data, allow us to build and analyse the spectral energydistributions (SEDs) of the compact sources in SDC335. Three HCHII regionswithin SDC335 are identified, two within the MM1 core. For each HCHII region, afree-free emission curve is fit to the data allowing the derivation of thesources' emission measure, ionising photon flux and electron density. Usingthese physical properties we assign each HCHII region a ZAMS spectral type,finding two protostars with characteristics of spectral type B1.5 and one witha lower limit of B1-B1.5. Ancillary data from infrared to mm wavelength areused to construct free-free component subtracted SEDs for the mm-cores,allowing calculation of the bolometric luminosities and revision of theprevious gas mass estimates. The measured luminosities for the two mm-cores arelower than expected from accreting sources displaying characteristics of theZAMS spectral type assigned to them. The protostars are still activelyaccreting, suggesting that a mechanism is limiting the accretion luminosity, wepresent the case for two different mechanisms capable of causing this. Finally,using the ZAMS mass values as lower limit constraints, a final stellarpopulation for SDC335 was synthesised finding SDC335 is likely to be in theprocess of forming a stellar cluster comparable to the Trapezium Cluster andNGC6334 I(N).

Tightening the belt: Constraining the mass and evolution in SDC335