High erucic acid oilseed rape (HEAR) was tested as a source crop for estimates of regional geneflow. Two methods to detect HEAR in low erucic acid oilseed rape (LEAR) were developed and compared. One used real-time quantitative PCR (qPCR) to detect the presence of HEAR fatty acid elongation genes in a LEAR genetic background; the other used gas chromatography (GC) to measure the rise in % erucic acid (EA) in seed of a HEAR-LEAR hybrid. In laboratory-mixed samples, qPCR detected 1/500 HEAR/LEAR seed, for LEAR varieties having a 2-base pair deletion in the gene; sensitivity was reduced for those varieties with a 4-base pair deletion. The GC method detected at least 1/500 for any combination of varieties. Field testing in Tayside and Hertfordshire juxtaposed 2.5 ha fields of a LEAR variety (0.028% EA) adjacent to and 1 km distant from a field of a HEAR variety (44% EA). The LEAR variety was a varietal association, having reduced self pollen, to ensure high cross pollination (CP). The methods were highly correlated measuring between 30% and 0.5% CP (0.97% variance accounted for). However, the qPCR method became unreliable around and below 0.5% CP (F1 seed in LEAR), whereas the GC method was robust enough to detect raised EA equivalent to one F1 seed in 500 (0.2%). To estimate low values of cross pollination, a statistical mixture model was fitted to the distributions of EA in samples in order to assign a CP value to each 500-seed sample. Declines of CP from 30% to <1%, and EA from 5% to 0.2%, with distance up to 150 m in the near fields was best fitted with a power-function. The combined mean EA for both far fields was 0.11%, well above the background LEAR value of 0.028%, and mean CP 0.36%. The GC method of detecting raised %EA should be a reliable and high-throughput means of estimating %CP between fields, provided the receptor seed is shown to be pure LEAR and the %EA of single F1 seed in receptor fields is measured to confirm the presence of F1 seed. Varietal associations with reduced self pollen will incur much higher % cross pollination from external fields than fully pollenfertile varieties.