  (Abridged) Studies of debris discs have shown that most systems are analogous
to the EKB. In this study we aim to determine how many IRAS 25um excesses
towards A stars are real, and investigate where the dust lies. We observe with
TIMMI2, VISIR, Michelle and TReCS a sample of A and B-type main sequence stars
reported as having mid-IR excess. We constrain the location of the debris
through combined modelling of the emission spectrum and a modelling technique
designed to constrain the radial extent of emission in mid-IR imaging. We
independently confirm the presence of warm dust around 3 of the candidates:
HD3003, HD80950 and eta Tel. For the binary HD3003 a stability analysis
indicates the dust is either circumstellar and lying at ~4 AU with the binary
orbiting at >14AU, or the dust lies in an unstable location; there is some
evidence for temporal evolution of its excess emission on a ~20 year timescale.
For 7 of the targets we present quantitative limits on the location of dust
around the star. We demonstrate that the disc around HD71155 must have
spatially distinct components at 2 and 60AU. We model the limits of current
instrumentation and show that most of the known A star debris discs which could
be readily resolved at 18um on 8m instruments have been resolved. Limits from
unresolved imaging can help distinguish between competing models of the disc
emission, but resolved imaging is key to the determination of the disc
location. Modelling of the detection limits for extended emission can be useful
for targeting future observational campaigns. MIRI on the JWST will be able to
resolve most of the known A star debris disc population. METIS on the E-ELT
will provide the opportunity to explore the hot disc population more thoroughly
by detecting extended emission where calibration accuracy limits disc detection
through photometry, reaching levels below 1 zodi for stars at <10pc.
