Jrtv's solution will only work if the Freeview box has a built-in RF modulator. Most Freeview boxes don't have this. (The 'RF out' on most Freeview boxes only provides 'loop-through'. i.e. it simply duplicates the analogue signal which is present in your aerial lead).
Some of the other solutions seem to be based upon the assumption that the 'AV' sockets on your TV are 'input' rather than 'output' sockets. (If so, you can buy a Scart to AV lead for a couple of quid from Tesco Extra). Unfortunately this seems to conflict with the information provided in your question.
If the only INPUT to the TV is via the aerial socket ('RF in'), then you'll have no choice (apart from replacing the TV) other than to get hold of an RF modulator. Unfortunately, as Rojash's second link shows, these cost nearly as much as the cheaper Freeview boxes.
If you know someone who could make use of your Freeview box (perhaps you've got a friend or relative with a Scart-enabled TV) it might be best to offer them your one at a slightly discounted price. You could then purchase another box, for yourself, which contains an RF modulator.
Chris