Like Spudqueen, I was thinking that �20 in 1976 was worth about 10% less than the weekly pay of someone paid at a 'basic' rate. That would make it worth about �180 today.
The price of a pint of bitter in 1976 was about 15p. Now it's about 15 times that price. So to buy the same quantity of beer would require �300 today.
If you do the same calculation based on the price of cigarettes, the multiplier is around 20. So, to buy the same quantity of cigarettes would cost about �400.
On the other hand, around 1976, I bought a 'cheap' digital watch. (It was one of the first LCD types). It was in a sale, reduced from �76 to �38. By that comparison, �20 in 1976 would have bought roughly half of a basic digital watch. Given that market traders sell digital watches for 99p, that means that �20 in 1976 equates to about 50p today!
50p or �400? It all depends how you work it out!
Chris