Can platelet count of 482 be caused by "bad" sinus infection (found on brain mri) and seasonal hayfever? Haem-.374, rcc-4.39, hameoglobin-12.5, mcv-85, mch-29.2, (iron-12, ferritin-30, %sat-17.7 in dec)

Maybe. That platelet count is very slightly high. It could certanly be due to the inflammation associate with a bad sinus infection. Or it could be where you normally run--it is not so high that on it's own I would be concerned. However, if it goes higher or if you ever have problems bleeding or clotting, you should consult with your doctor.
Platlets. Platlets are a poor mans sed rate they are an indicator sometimes of an infection or inflamation a platlet count as a solo criteria of illness is not something to worry about many factors plus a high platlet count matters more.