I have a rattle/knock from the front suspension on rough ground at low speed approx. 10mph

The forum has a really good search facility. It's found by clicking on the "search" link at the top of the page. There's a whole sticky thread on suspension knock

You'll probably find that either your anti roll bar bushes (ARB bushes) or your lower front control arm (wishbone) bushes are shot and in need of replacement. The ARB bushes are cheapest so try those first. There are other causes of noise but these seem to be the most common. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

Seeing as you have a TDV8, they all came with ARC, and the bushes are made from cheese...

Around 60-70k miles though, the front arm bushes may be getting a bit tired.

Agreed. My ARB bushes were replaced at about 50k miles and the lower wishbones recently at about 63k miles. Rear ARB bushes were done when the rear ARB was replaced at about 50k miles. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

Mine just had an MOT. Yeah
Rear n/s and O/s track control arms had minimal movement in the ball joints according to advisory. Bit confused dot com as the tyres are wearing dead flat and from what I can see, the track control arms have bushes both ends and no ball joints. Might need to clarify that with Lexus dealer when I next go over as dealer uses them for Mot's.  2x TDV6. SE and HSE. Both returned to sender with issues👀
TDV8 Sport HSE...... Gone. Loved it but it didn't love me!!
Now rolling in MY10 Fullfat TDV8 Vogue. Slammed it to go round corners🙈
Cruising daily #56 mph in a Volvo V8. 😱
Lots of other toys in the cupboard...😜

The ball joints are on the inner ends, attached to the chassis