Magpies are persistently trying to raid a blackbird’s nest in my garden. One made off with an unhatched chick or fledgling yesterday, but the parents are returning to the nest so I assume some survived. The nest is in thick climber on top of trellis. I’m w

This is always distressing to witness and may persist as now the Magpie has found the nest it could well return for the rest of the eggs/chicks ... the hardest part is that there's nothing you can do as it's against wildlife laws to interfere with an active nest ... nature at it's most gruesome

Thankyou for your advice, Wendy. (And sympathy!) I just meant draping some netting over the top of the climber, but I imagine a magpie could find its way in anyway. 

The male blackbird seems to be being more assertive in warding off the magpie today 

Any actions in trying to protect nest could result in Blackbird abandoning the nest ... they are very feisty in trying to ward off predators though not always successfully ... I can remember watching many a 'dog fight' when I had Kestrels nesting in my quarry, constant aerial attacks by Blackbirds when the youngsters were roaming too close to their nests and they often made physical contact which made the Kestrel youngsters cry out!

Noted, thankyou. Interesting about the kestrels. 

I remember seeing a goshawk being mobbed/chased by multiple blackbirds.

I suspect the blackbirds are still in that location (somewhere in the Peak District). But the goshawks may not be.