I see signs of chlorosis which could indicate high alkalinity, compacted roots & nutrient deficiency. I would repot & to a much larger pot & new potting medium made specifically for acid loving plants. Keep soil consistently moist, bright shade.

Don't use ericaceous compost, Hebe aren't too fussy about soil ph, will grow in alkaline and neutral soil, so other than disliking anything more than just slightly acidic,

(see here http://www.hebesoc.org/hebes_cultivation/hebes_cultivation.htm)

soil ph isn't the issue. Otherwise, yes, it either needs a larger pot or planting in the ground. LIkely the growth problems and yellowing are due to insufficient root room.

A feed wouldn't hurt but evergreens do tend to lose leaves too. One problem I have found with hebes is that they can get a bit leggy and lose lower leaves. Perhaps it might not go a miss to give a light trim after flowering to keep it bushy.

Thanks- will pot up into bigger pot. Another job for the weekend to do list. Weather looks good so catching up on deadheading and injecting some colour into the garden.