There is a few ways you can record reimbursements in Coconut.
With an invoice
When invoicing your client remember to send invoice for both the expense and the work. You can do that by including multiple line items on your invoice.
When the payment for the invoice comes in, record the entire payment in as income and either:
categorise the correct outgoing transaction as an expense, or
add manual transaction to record the amount for the expense
Invoice for £1200 with line items for:
£1000 for work
£200 for travel (train ticket)
Payment for the invoice, for the total of £1200, arrives to your account and you classify this entire transaction as income. For the expenses, you either find and re-categorise the transaction for the train ticket as an expense or you add a manual transaction for £200 and categorise it as an expense.
Without an invoice
When the payment for work and expenses together arrives into your account, make sure you have already correctly categorised the payment out for the expense or added a manual transaction to reflect the expense. Then, record the total amount that you were paid as income.
You receive £1200 payment, inclusive of £1000 for work and £200 reimbursement for train tickets you have purchased before. You need to classify this transaction as income and either find and re-categorise the transaction for the train ticket as an expense or you add a manual transaction for £200 and categorise it as an expense.
In both cases you will report the total paid to you inclusive of the reimbursements as turnover to HMRC, and report the expense that you go on to claim as an expense.
Receiving reimbursement as a separate transaction
If your client is sending you reimbursement as a separate transaction you can categorise it as Income > Reimbursements. That way the reimbursement won't be included in your income totals and won't count towards your tax savings suggestion.