  Follow-up observations of (sub-)mm-selected gravitationally-lensed systems
have allowed a more detailed study of the dust-enshrouded phase of
star-formation up to very early cosmic times. Here, the case of the
gravitationally lensed merger in HATLAS J142935.3-002836 (also known as
H1429-0028; z_lens=0.218, z_bkg=1.027) is revisited following recent
developments in the literature and new APEX observations targeting two carbon
monoxide (CO) rotational transitions J_up=3 and 6. We show that the
line-profiles comprise three distinct velocity components, where the fainter
high-velocity one is less magnified and more compact. The modelling of the
observed spectral line energy distribution of CO J_up=2 to 6 and [CI]3P_1-3P_0
assumes a large velocity gradient scenario, where the analysis is based on four
statistical approaches. Since the detected gas and dust emission comes
exclusively from only one of the two merging components (the one oriented
North-South, NS), we are only able to determine upper-limits for the companion.
The molecular gas in the NS component in H1429-0028 is found to have a
temperature of ~70K, a volume density of log(n[/cm3])~3.7, to be expanding at
~10km/s/pc, and amounts to M_H2=4(-2,+3)*1e9 Msun. The CO to H2 conversion
factor is estimated to be alpha_CO=0.4(-0.2,+0.3) Msun/(K.km/s.pc2). The NS
galaxy is expected to have a factor of >10x more gas than its companion
(M_H2<3e8 Msun). Nevertheless, the total amount of molecular gas in the system
comprises only up to 15 per cent (1sigma upper-limit) of the total (dynamical)
mass.
