The son of a British couple detained in Tehran on espionage charges has called on Keir Starmer to prioritise their case in the “very opportune moment” of a ceasefire in the Iran conflict.

Lindsay and Craig Foreman, from East Sussex, were arrested while on a five-day trip across Iran in January last year and have been held in Evin prison for 15 months.

As Starmer visits the Gulf to discuss efforts to uphold the ceasefire, the couple’s family have called on the prime minister and his foreign secretary, Yvette Cooper, to raise the case in all diplomatic discussions.

“If peace is genuinely part of the agenda, then that has to include the releasing of innocent people. I don’t see how it can be peaceful and that’s the case,” said Lindsay’s son, Joe Bennett.

The 31-year-old said it was a “very opportune moment” for the prime minister and the government should “take courage” from the release this week of two French citizens. Cécile Kohler and Jacques Paris had been detained in Iran for almost four years on espionage charges and were also held in Evin prison before they were released but barred from leaving Tehran.

“Macron was incredibly vocal, he was championing above all else, and I think that’s where we can take some courage and have a look at how do we apply that kind of pressure in the right way,” said Bennett. “There’s a genuine opportunity for resolution here.”

The UK government has faced claims that it is in denial about the extent of hostage-taking by foreign states and has failed to fulfil its promise to appoint a special envoy to combat the issue.

Bennett has been campaigning for the couple’s release with significant cross-party parliamentary support. Despite several meetings with the foreign secretary, and a hearing into their case held by the all-party parliamentary group on arbitrary detention and hostage affairs, he said there has been no visible progress in 15 months.

On Thursday morning, Bennett spoke with his mother in Evin prison. The past 48 hours had been tumultuous, he said, after a supposed ceasefire was agreed on Tuesday little more than an hour before a self-imposed deadline by Donald Trump, who had threatened Iran’s “whole civilisation” would die if it did not meet his demands.

The comments made Bennett feel “pure fear” until news of the two-week ceasefire was announced. The potential for peace talks has given the couple some hope, he said, that “they’ll be fought for” by the UK government.

“Yvette Cooper called it unjustifiable and that she would be fighting relentlessly to get them home. I need to see signs of that,” said Bennett.

In his communications with the Foreign Office, which has been approached for comment, Bennett said he has been repeatedly told the situation was being assessed on the ground. But there was no accountability, or plan, he said, adding that three weeks ago he had had to push for virtual consular assistance.

“There’s a genuine opportunity here, we can hopefully make progress,” said Bennett. “And that’s not just on the strait of Hormuz, that’s not just on oil prices – everyone talks about the top of the agenda but they forget that there’s innocent human beings at the centre of this.”