Hi.

We have a 5 month old Golden Retriever puppy who currently has 2 20 min walks a day with half of this being off lead maybe having a play with another dog if we meet one.

Does this sound about right as I am very concious of over doing it on her delicate joints.

The rule of thumb is generally 5 minutes per month old 2 times per day, so 5 months x 5 mins = 25mins twice a day, with ample resting time in between. This is of lead walking etc. I generally don't let young pups rough house with adult dogs, unless they are a similar weight and size, as naturally pups always seem to try and keep up with the big ones.  I always keep free running to one pup alone, so that its done at their pace, rather than them competing with the older ones in races etc.
Everything basically should be in moderation. With ample rest time in between playing. I also restrict the amount of ball throwing etc so they aren't suddenly stopping turning etc. Maybe 3 throws per game etc.

We will increase to 25 mins now then she could do with the extra 10 mins a day,she is full of it!! She tends to get walked at 9 in the morn and 7.30 at night so that is a good rest in between. We do a bit of ball throwing,but not too far or fast just to work on her retrieval which is now very good(well for a treat!!).

Providing she's nothing like my BC on a ball chase that sounds fine. The obvious difference between my bc and your retriever is our ball is all about the chase (all though he does bring it back :-D ) and he turns on a penny from a full pelt gallop. But for a retriever I imagine the satisfaction they get from the job is taking it back rather than chasing after it as it flies, which is why they are probably a little more gentle in their approach to getting the ball.

I am walkin my two 6 month old pups for about 30 mins aday, that is fine for them, they have the run of a back garden for most of the day!