Only 15 of the 52 are actually practising mainstream economists:

Howard Reed, Landman Economics; – Tax specialist
Prof Geoffrey M Hodgson, University of Hertfordshire – Business Studies
Professor Mariana Mazzucato, Open University – Economics of Innovation
Professor Marcus Miller, University of Warwick – Macroeconomics
Professor Dennis Leech, University of Warwick – Economics
Prof. Giuseppe Fontana, Leeds University – Monetary Economics
Professor Susan Himmelweit, Open University – Economics
Prof Malcolm Sawyer, University of Leeds – Economics
Dr Paul Segal, University of Sussex – Economics
Dr Jonathan Perraton, University of Sheffield – Economics
Ismail Erturk, University of Manchester – Banking
Professor Matthew Watson, University of Warwick – Political Economy
Professor Sir Tony Atkinson, Nuffield College, Oxford – Public Economics
Professor Andy Danford, Bristol Business School (Research Director)
Professor Gregor Gall, University of Hertfordshire – Industrial Relations

Quite a few are active campaigners:

Andrew Watt, Senior Researcher, European Trade Union Institute
Michael Burke, Economic Consultant (frequent Guardian columnist)
Pat Devine, University of Manchester – Industrial Economics (Google says he's a 'Radical economist')
Richard Murphy, Director, Tax Research LLP – Anti-Poverty Campaigner

Including several from the left-wing new economics foundation:

James Meadway, Senior economist, new economics foundation
Ruth Potts, Campaign Manager, the Great Transition (at the new economics foundation)
Andrew Simms, nef fellow and Green New Deal Group Member
James Meadway, Senior economist, new economics foundation

A good many of those signing the letter are no longer active in academe:

Sheila Dow, University of Stirling – Economics (retired)
Barbara MacLennan, Universities of York and Manchester – Economics (retired)
Professor Derek Braddon, University of the West of England – Economics (retired)
Ian Gough, Emeritus Professor, University of Bath – Social and Policy Studies (retired)
David Purdy, University of Manchester (retired)
lan O'Shea, UEL – Cultural Studies (retired)

Several more seem to have more overseas than UK experience:

Henning Meyer, LSE – Global Governance;
Robin Murray, Senior Visiting Fellow, Global Governance, LSE
Prof George Irvin, Univ of London, SOAS – Development Studies
Professor Diane Elson, University of Essex – Development Studies
Professor David Bailey, Coventry University – International Business Strategy
Jonathan Glennie, Overseas Development Institute (former Christian Aid manager)

A large number are do not teach economics but 'organizational studies' and similar subjects:

Dr Gregory Schwartz, University of Bath – Organizational Studies
Professor Alison Pullen, Swansea University – Organization Studies
Dr Damian O'Doherty, University of Manchester – People, Management and Organizations
Professor Simon Lilley, University of Leicester – Information and Organization
Colin Crouch, University of Warwick – Governance and Public Management
Nick Isles, Managing Director of Corporate Agenda – Organizational Performance
Professor Stephen Haseler, Global Policy Institute – Constitution and International Relations
Prof Peter Case, Bristol Business School – Leadership and Organizational Ethics

While several more are social policy theory:

Professor Adrian Sinfield, University of Edinburgh – Social Policy
Professor Stephen Linstead, University of York – Organizational Theory Research
David Donald, Glasgow Caledonian University – Political Science
Professor David Marquand, Oxford University – Politics (former Labour MP)
Stuart White, Jesus College, Oxford University – Politics
Valerie Bryson, Emerita Professor of Politics, University of Huddersfield – Politics (retired)
Alan Finlayson, Reader, Dept. of Political and Cultural Studies , Swansea – Political Theory

Some are historians, others involved in culture, and even media studies:

Prof Richard Grayson, Goldsmiths, University of London – History
Professor Jonathan Rutherford, Middlesex University – Cultural Studies
Professor Stefano Harney, Queen Mary, University of London – Strategy, Culture and Society
Professor David Knights, Bristol Business School – Ethics, Gender Studies, Financialization
Professor Natalie Fenton, Goldsmiths, University of London – Media and Communications
Dr. Douglas Chalmers, Glasgow Caledonian University – Media and Journalism

A few hardly register on Google at all:

Mark Fisher, University of London
Stewart Lansley, Research Fellow, Bristol University
Dr. Olivier Ratle, University of the West of England, Bristol

So: a devastating broadside that will blow a hole in the government's economic strategy? Hardly.