This paper is part of a series discussing the results obtained in the framework of a wide international collaboration -the Lockman Hole Project -aimed at improving the extensive multi-band coverage available in the Lockman Hole region, through novel deep, wide-area, multi-frequency (60, 150, 350 MHz and 1.4 GHz) radio surveys. This multi-frequency, multiband information will be exploited to get a comprehensive view of star formation and AGN activities in the high redshift Universe from a radio perspective. In this paper we present novel 1.4 GHz mosaic observations obtained with the Westerbork Synthesis Radio Telescope (WSRT). With an area coverage of 6.6 square degrees, this is the largest survey reaching an rms noise of 11 µJy/b. In this paper we present the source catalogue (∼ 6000 sources with flux densities S > ∼ 55 µJy (5σ), and we discuss the 1.4 GHz source counts derived from it. Our source counts provide very robust statistics in the flux range 0.1 < S < 1 mJy, and are in excellent agreement with other robust determinations obtained at lower and higher flux densities. A clear excess is found with respect to the counts predicted by the semi-empirical radio sky simulations developed in the framework of the SKA Simulated Skies project. A preliminary analysis of the identified (and classified) sources suggests this excess is to be ascribed to star forming galaxies, which seem to show a steeper evolution than predicted.

We have studied a sample of 296 faint (> 0.5 mJy) radio sources selected from an area of the Tenth Cambridge (10C) survey at 15.7 GHz in the Lockman Hole. By matching this catalogue to several lower frequency surveys (e.g. including a deep GMRT survey at 610 MHz, a WSRT survey at 1.4 GHz, NVSS, FIRST and WENSS) we have investigated the radio spectral properties of the sources in this sample; all but 30 of the 10C sources are matched to one or more of these surveys. We have found a significant increase in the proportion of flat spectrum sources at flux densities below ≈1 mJy -the median spectral index between 15.7 GHz and 610 MHz changes from 0.75 for flux densities greater than 1.5 mJy to 0.08 for flux densities less than 0.8 mJy. This suggests that a population of faint, flat spectrum sources is emerging at flux densities 1 mJy.The spectral index distribution of this sample of sources selected at 15.7 GHz is compared to those of two samples selected at 1.4 GHz from FIRST and NVSS. We find that there is a significant flat spectrum population present in the 10C sample which is missing from the samples selected at 1.4 GHz. The 10C sample is compared to a sample of sources selected from the SKADS Simulated Sky by Wilman et al. and we find that this simulation fails to reproduce the observed spectral index distribution and significantly underpredicts the number of sources in the faintest flux density bin. It is likely that the observed faint, flat spectrum sources are a result of the cores of FRI sources becoming dominant at high frequencies.These results highlight the importance of studying this faint, high frequency population.