The largely unexplored decametre radio band (10-30MHz) provides a unique window for studying a range of astronomical topics, such as auroral emission from exoplanets, inefficient cosmic ray acceleration mechanisms and fossil radio plasma. The scarcity of low-frequency studies is mainly due to severe ionospheric corruption. Here we present a calibration strategy to correct for the ionosphere in the decametre band. We apply this to an observation from the Low-Frequency Array (LOFAR) between 16 and 30MHz. The resulting image covers 330 square degrees of sky at a resolution of 45", reaching a sensitivity of 12mJy per beam, which is an improvement by an order of magnitude in terms of sensitivity and resolution compared to previous decametre observations. Residual ionospheric effects cause additional blurring between 60" and 100". We have identified four fossil plasma sources in the surveyed region. These sources probably harbour rejuvenated radio plasma from past active galactic nuclei outbursts. Three are near the centre of low-mass galaxy clusters. Notably, two of these sources display the steepest radio spectral index among all the sources detected at 23MHz. This indicates that fossil plasma sources constitute the primary population of steep-spectrum sources at these frequencies, emphasizing the large discovery potential of ground-based decametre observations.