I haven't heard that the Waxwings are back but you are right, Johnson's Park is usually the best place to see them. Last year there were a lot of twitchers turning up over a period of around 2 months, some hung apples and other tasty morsels in the tree outside the entrance to the park to tempt them in, it seemed to work really well! If you do get to see them and manage to get any photos I'd love to see them uploaded to a gallery. Good luck :)
They aren't there yet I don't think, but if you're interested in birds Yeovil is home to some truly amazing Starling murmurations. My favourite spot for watching the ebb and flow of this wonderful spectacle is as follows:
As you come into Yeovil from the dual carriageway (A30) take the third exit towards the farm you see (instead of going straight on towards Pittards/Mole Valley etc). The farm is on your left as you go up the narrow road (only need to go about 20 metres)
There is a small siding just after the farm buildings where you can pull in and look over the fields. I have seen some of the best murmurations here. Hope this helps. Ron.