Flavour is an important aspect affecting lettuce (Lactuca sativa) consumption. Lettuce production is a $1.9bn industry in the USA (FAOstat). The flavour is mostly a balance of desirable sugars and secondary metabolites such as the undesirable sesquiterpene lactones (SLs) which are characteristic compounds of asteraceous plants. They are largely responsible for the bitter taste often experienced when consuming lettuce, as they are produced by the plant as anti-feedants, in addition to their role as phytoalexins. Conversely high sugar content counteracts the bitterness of the SLs, but makes the plant a greater target for pathogens and chewing insects. We hypothesise that the SL profile could be manipulated to bring a less bitter variety to the market without impacting on the ability of the plant to survive environmental stress. We present NMR spectroscopy data used to determine the relative concentrations of SL compounds and other secondary plant metabolites across a mapping population of 95 F10 RILs derived from a L. sativa cv. Salinas x L. serriola parental cross, grown over a number of trials in the UK; a standard field trial alongside a nitrogen deficiency field trial (Cobham, UK), and a controlled environment trial (Reading, UK) in order to identify genetically stable QTL over environmental interactions. The QTL derived from NMR have been compared to QTL obtained from reference datasets. LCMS using a fluorescence detector was used to identify and quantify the SLs lactucin-15-oxalate, 15-p-hydroxyphenylacetyllactucin-8-sulphate, and 8-deoxylactucin; total antioxidants were measured using the ferric reducing ability of plasma (FRAP), and the sugars glucose, sucrose and fructose were quantified using an enzyme assay.