The police drama, which first aired in 1983, came in ahead of BBC One period drama 'Casualty 1909', which brought in 3.3m viewers and a 14.2% share in the same 9pm-10pm slot.
BBC Two's Antarctica adventure documentary staring Ben Fogle, James Cracknell and Ed Coats followed closely behind with 3.2m viewers and a 13.7% share.
Channel 4's Big Brother, now in its tenth series, managed to attract 2.1m viewers and a 9.2% share, with a further 296,000 tuning in on Channel 4+1.
Five's 8pm movie, 'Practical Magic' starring Nicole Kidman and Sandra Bullock, notched up 927,000 viewers and a 3.9% share over its two-hour slot.
Throughout Saturday evening ITV1 lagged its main publicly-funded rival, excepting its 3.6m audience for teatime hidden camera programme 'Anonymous'.
Featuring celebrities such as 'X Factor' judge Louis Walsh donning prosthetic masks to fool their friends, 'Anonymous' gave ITV1 a 21.9% share at 6.30pm-7.30pm.
This was well ahead of 6.30pm-7.15pm BBC One game show 'Totally Saturday', which limped on with 2m viewers and a 12.3% share.
But BBC One stormed ahead with obstacle course game show 'Total Wipeout', which brought in 4.5m viewers and a 24.6% share at 7.15pm-8.15pm compared with 'Who Wants To Be A Millionaire?'s 3.4m viewers and 18.4% share at 7.30pm-8.30pm.
The gap widened between 8.30pm and 10.30pm when 'A Touch Of Frost' brought in 3.1m viewers, a 15.9% share.
BBC One beat this first with 4.9m viewers for 'The National Lottery: Guesstimation' at 8.15pm-9.05pm and then 5.1m viewers for 'Casualty' at 9.05pm-9.55pm.
BBC Two put in the strongest showing out of the other channels with 2.2m viewers at 8.15pm-9.15pm -- an 11.1% share -- for a documentary about the palace of Versailles.
On Friday night 'Big Brother' managed 2.7m viewers for an eviction special which dumped out housemate Carly as well as introducing five new housemates.
This gave it a 12.4% share at 9pm-10pm, behind ITV1's 18.5% share for 'Doc Martin' and BBC One's 15% share for 'Hotel Bablyon'.
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